# Attention

Incorporating attention mechanisms to create context-aware representations.
Goku Mohandas
· · ·
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## Overview

In the RNN lesson, we were constrained to using the representation at the very end but what if we could give contextual weight to each encoded input ($$h_i$$) when making our prediction? This is also preferred because it can help mitigate the vanishing gradient issue which stems from processing very long sequences. Below is attention applied to the outputs from an RNN. In theory, the outputs can come from anywhere where we want to learn how to weight amongst them but since we're working with the context of an RNN from the previous lesson , we'll continue with that.

$\alpha = softmax(W_{attn}h)$
$c_t = \sum_{i=1}^{n} \alpha_{t,i}h_i$

Variable Description
$$N$$ batch size
$$M$$ max sequence length in the batch
$$H$$ hidden dim, model dim, etc.
$$h$$ RNN outputs (or any group of outputs you want to attend to) $$\in \mathbb{R}^{NXMXH}$$
$$\alpha_{t,i}$$ alignment function context vector $$c_t$$ (attention in our case) $$W_{attn}$$ attention weights to learn $$\in \mathbb{R}^{HX1}$$ $$c_t$$ context vector that accounts for the different inputs with attention • Objective: • At it's core, attention is about learning how to weigh a group of encoded representations to produce a context-aware representation to use for downstream tasks. This is done by learning a set of attention weights and then using softmax to create attention values that sum to 1. • Advantages: • Learn how to account for the appropriate encoded representations regardless of position. • Disadvantages: • Another compute step that involves learning weights. • Miscellaneous: • Several state-of-the-art approaches extend on basic attention to deliver highly context-aware representations (ex. self-attention). ## Set up Let's set our seed and device for our main task.  1 2 3 4 5 6 import numpy as np import pandas as pd import random import torch import torch.nn as nn import torch.nn.functional as F   1 SEED = 1234   1 2 3 4 5 6 7 def set_seeds(seed=1234): """Set seeds for reproducibility.""" np.random.seed(seed) random.seed(seed) torch.manual_seed(seed) torch.cuda.manual_seed(seed) torch.cuda.manual_seed_all(seed) # multi-GPU   1 2 # Set seeds for reproducibility set_seeds(seed=SEED)   1 2 3 4 5 6 7 8 # Set device cuda = True device = torch.device("cuda" if ( torch.cuda.is_available() and cuda) else "cpu") torch.set_default_tensor_type("torch.FloatTensor") if device.type == "cuda": torch.set_default_tensor_type("torch.cuda.FloatTensor") print (device)  cuda  ### Load data We will download the AG News dataset, which consists of 120K text samples from 4 unique classes (Business, Sci/Tech, Sports, World)  1 2 3 4 5 # Load data url = "https://raw.githubusercontent.com/GokuMohandas/Made-With-ML/main/datasets/news.csv" df = pd.read_csv(url, header=0) # load df = df.sample(frac=1).reset_index(drop=True) # shuffle df.head()  title category 0 Sharon Accepts Plan to Reduce Gaza Army Operation... World 1 Internet Key Battleground in Wildlife Crime Fight Sci/Tech 2 July Durable Good Orders Rise 1.7 Percent Business 3 Growing Signs of a Slowing on Wall Street Business 4 The New Faces of Reality TV World ### Preprocessing We're going to clean up our input data first by doing operations such as lower text, removing stop (filler) words, filters using regular expressions, etc.  1 2 3 4 import nltk from nltk.corpus import stopwords from nltk.stem import PorterStemmer import re   1 2 3 4 nltk.download("stopwords") STOPWORDS = stopwords.words("english") print (STOPWORDS[:5]) porter = PorterStemmer()  [nltk_data] Downloading package stopwords to /root/nltk_data... [nltk_data] Package stopwords is already up-to-date! ['i', 'me', 'my', 'myself', 'we']    1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 def preprocess(text, stopwords=STOPWORDS): """Conditional preprocessing on our text unique to our task.""" # Lower text = text.lower() # Remove stopwords pattern = re.compile(r"\b(" + r"|".join(stopwords) + r")\b\s*") text = pattern.sub("", text) # Remove words in parenthesis text = re.sub(r"$$[^)]*$$", "", text) # Spacing and filters text = re.sub(r"([-;;.,!?<=>])", r" \1 ", text) text = re.sub("[^A-Za-z0-9]+", " ", text) # remove non alphanumeric chars text = re.sub(" +", " ", text) # remove multiple spaces text = text.strip() return text   1 2 3 # Sample text = "Great week for the NYSE!" preprocess(text=text)  great week nyse   1 2 3 4 # Apply to dataframe preprocessed_df = df.copy() preprocessed_df.title = preprocessed_df.title.apply(preprocess) print (f"{df.title.values[0]}\n\n{preprocessed_df.title.values[0]}")  Sharon Accepts Plan to Reduce Gaza Army Operation, Haaretz Says sharon accepts plan reduce gaza army operation haaretz says  Warning If you have preprocessing steps like standardization, etc. that are calculated, you need to separate the training and test set first before applying those operations. This is because we cannot apply any knowledge gained from the test set accidentally (data leak) during preprocessing/training. However for global preprocessing steps like the function above where we aren't learning anything from the data itself, we can perform before splitting the data. ### Split data  1 2 import collections from sklearn.model_selection import train_test_split   1 2 3 TRAIN_SIZE = 0.7 VAL_SIZE = 0.15 TEST_SIZE = 0.15   1 2 3 4 5 def train_val_test_split(X, y, train_size): """Split dataset into data splits.""" X_train, X_, y_train, y_ = train_test_split(X, y, train_size=TRAIN_SIZE, stratify=y) X_val, X_test, y_val, y_test = train_test_split(X_, y_, train_size=0.5, stratify=y_) return X_train, X_val, X_test, y_train, y_val, y_test   1 2 3 # Data X = preprocessed_df["title"].values y = preprocessed_df["category"].values   1 2 3 4 5 6 7 # Create data splits X_train, X_val, X_test, y_train, y_val, y_test = train_val_test_split( X=X, y=y, train_size=TRAIN_SIZE) print (f"X_train: {X_train.shape}, y_train: {y_train.shape}") print (f"X_val: {X_val.shape}, y_val: {y_val.shape}") print (f"X_test: {X_test.shape}, y_test: {y_test.shape}") print (f"Sample point: {X_train[0]} โ {y_train[0]}")  X_train: (84000,), y_train: (84000,) X_val: (18000,), y_val: (18000,) X_test: (18000,), y_test: (18000,) Sample point: china battles north korea nuclear talks โ World  ### Label encoding Next we'll define a LabelEncoder to encode our text labels into unique indices  1 import itertools    1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 class LabelEncoder(object): """Label encoder for tag labels.""" def __init__(self, class_to_index={}): self.class_to_index = class_to_index or {} # mutable defaults ;) self.index_to_class = {v: k for k, v in self.class_to_index.items()} self.classes = list(self.class_to_index.keys()) def __len__(self): return len(self.class_to_index) def __str__(self): return f"" def fit(self, y): classes = np.unique(y) for i, class_ in enumerate(classes): self.class_to_index[class_] = i self.index_to_class = {v: k for k, v in self.class_to_index.items()} self.classes = list(self.class_to_index.keys()) return self def encode(self, y): encoded = np.zeros((len(y)), dtype=int) for i, item in enumerate(y): encoded[i] = self.class_to_index[item] return encoded def decode(self, y): classes = [] for i, item in enumerate(y): classes.append(self.index_to_class[item]) return classes def save(self, fp): with open(fp, "w") as fp: contents = {'class_to_index': self.class_to_index} json.dump(contents, fp, indent=4, sort_keys=False) @classmethod def load(cls, fp): with open(fp, "r") as fp: kwargs = json.load(fp=fp) return cls(**kwargs)   1 2 3 4 5 # Encode label_encoder = LabelEncoder() label_encoder.fit(y_train) NUM_CLASSES = len(label_encoder) label_encoder.class_to_index  {'Business': 0, 'Sci/Tech': 1, 'Sports': 2, 'World': 3}   1 2 3 4 5 6 # Convert labels to tokens print (f"y_train[0]: {y_train[0]}") y_train = label_encoder.encode(y_train) y_val = label_encoder.encode(y_val) y_test = label_encoder.encode(y_test) print (f"y_train[0]: {y_train[0]}")  y_train[0]: World y_train[0]: 3   1 2 3 4 # Class weights counts = np.bincount(y_train) class_weights = {i: 1.0/count for i, count in enumerate(counts)} print (f"counts: {counts}\nweights: {class_weights}")  counts: [21000 21000 21000 21000] weights: {0: 4.761904761904762e-05, 1: 4.761904761904762e-05, 2: 4.761904761904762e-05, 3: 4.761904761904762e-05}  ### Tokenizer We'll define a Tokenizer to convert our text input data into token indices.  1 2 3 import json from collections import Counter from more_itertools import take    1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 class Tokenizer(object): def __init__(self, char_level, num_tokens=None, pad_token="", oov_token="", token_to_index=None): self.char_level = char_level self.separator = "" if self.char_level else " " if num_tokens: num_tokens -= 2 # pad + unk tokens self.num_tokens = num_tokens self.pad_token = pad_token self.oov_token = oov_token if not token_to_index: token_to_index = {pad_token: 0, oov_token: 1} self.token_to_index = token_to_index self.index_to_token = {v: k for k, v in self.token_to_index.items()} def __len__(self): return len(self.token_to_index) def __str__(self): return f"" def fit_on_texts(self, texts): if not self.char_level: texts = [text.split(" ") for text in texts] all_tokens = [token for text in texts for token in text] counts = Counter(all_tokens).most_common(self.num_tokens) self.min_token_freq = counts[-1][1] for token, count in counts: index = len(self) self.token_to_index[token] = index self.index_to_token[index] = token return self def texts_to_sequences(self, texts): sequences = [] for text in texts: if not self.char_level: text = text.split(" ") sequence = [] for token in text: sequence.append(self.token_to_index.get( token, self.token_to_index[self.oov_token])) sequences.append(np.asarray(sequence)) return sequences def sequences_to_texts(self, sequences): texts = [] for sequence in sequences: text = [] for index in sequence: text.append(self.index_to_token.get(index, self.oov_token)) texts.append(self.separator.join([token for token in text])) return texts def save(self, fp): with open(fp, "w") as fp: contents = { "char_level": self.char_level, "oov_token": self.oov_token, "token_to_index": self.token_to_index } json.dump(contents, fp, indent=4, sort_keys=False) @classmethod def load(cls, fp): with open(fp, "r") as fp: kwargs = json.load(fp=fp) return cls(**kwargs)  Warning It's important that we only fit using our train data split because during inference, our model will not always know every token so it's important to replicate that scenario with our validation and test splits as well.  1 2 3 4 5 # Tokenize tokenizer = Tokenizer(char_level=False, num_tokens=5000) tokenizer.fit_on_texts(texts=X_train) VOCAB_SIZE = len(tokenizer) print (tokenizer)  <Tokenizer(num_tokens=5000)>   1 2 3 # Sample of tokens print (take(5, tokenizer.token_to_index.items())) print (f"least freq token's freq: {tokenizer.min_token_freq}") # use this to adjust num_tokens  [('<PAD>', 0), ('<UNK>', 1), ('39', 2), ('b', 3), ('gt', 4)] least freq token's freq: 14   1 2 3 4 5 6 7 8 # Convert texts to sequences of indices X_train = tokenizer.texts_to_sequences(X_train) X_val = tokenizer.texts_to_sequences(X_val) X_test = tokenizer.texts_to_sequences(X_test) preprocessed_text = tokenizer.sequences_to_texts([X_train[0]])[0] print ("Text to indices:\n" f" (preprocessed) โ {preprocessed_text}\n" f" (tokenized) โ {X_train[0]}")  Text to indices: (preprocessed) โ china battles north korea nuclear talks (tokenized) โ [ 16 1491 285 142 114 24]  ### Padding We'll need to do 2D padding to our tokenized text.  1 2 3 4 5 6 7 def pad_sequences(sequences, max_seq_len=0): """Pad sequences to max length in sequence.""" max_seq_len = max(max_seq_len, max(len(sequence) for sequence in sequences)) padded_sequences = np.zeros((len(sequences), max_seq_len)) for i, sequence in enumerate(sequences): padded_sequences[i][:len(sequence)] = sequence return padded_sequences   1 2 3 4 # 2D sequences padded = pad_sequences(X_train[0:3]) print (padded.shape) print (padded)  (3, 6) [[1.600e+01 1.491e+03 2.850e+02 1.420e+02 1.140e+02 2.400e+01] [1.445e+03 2.300e+01 6.560e+02 2.197e+03 1.000e+00 0.000e+00] [1.200e+02 1.400e+01 1.955e+03 1.005e+03 1.529e+03 4.014e+03]]  ### Datasets We're going to create Datasets and DataLoaders to be able to efficiently create batches with our data splits.   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 class Dataset(torch.utils.data.Dataset): def __init__(self, X, y): self.X = X self.y = y def __len__(self): return len(self.y) def __str__(self): return f"" def __getitem__(self, index): X = self.X[index] y = self.y[index] return [X, len(X), y] def collate_fn(self, batch): """Processing on a batch.""" # Get inputs batch = np.array(batch) X = batch[:, 0] seq_lens = batch[:, 1] y = batch[:, 2] # Pad inputs X = pad_sequences(sequences=X) # Cast X = torch.LongTensor(X.astype(np.int32)) seq_lens = torch.LongTensor(seq_lens.astype(np.int32)) y = torch.LongTensor(y.astype(np.int32)) return X, seq_lens, y def create_dataloader(self, batch_size, shuffle=False, drop_last=False): return torch.utils.data.DataLoader( dataset=self, batch_size=batch_size, collate_fn=self.collate_fn, shuffle=shuffle, drop_last=drop_last, pin_memory=True)    1 2 3 4 5 6 7 8 9 10 11 12 # Create datasets train_dataset = Dataset(X=X_train, y=y_train) val_dataset = Dataset(X=X_val, y=y_val) test_dataset = Dataset(X=X_test, y=y_test) print ("Datasets:\n" f" Train dataset:{train_dataset.__str__()}\n" f" Val dataset: {val_dataset.__str__()}\n" f" Test dataset: {test_dataset.__str__()}\n" "Sample point:\n" f" X: {train_dataset[0][0]}\n" f" seq_len: {train_dataset[0][1]}\n" f" y: {train_dataset[0][2]}")  Datasets: Train dataset: <Dataset(N=84000)> Val dataset: <Dataset(N=18000)> Test dataset: <Dataset(N=18000)> Sample point: X: [ 16 1491 285 142 114 24] seq_len: 6 y: 3    1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 # Create dataloaders batch_size = 64 train_dataloader = train_dataset.create_dataloader( batch_size=batch_size) val_dataloader = val_dataset.create_dataloader( batch_size=batch_size) test_dataloader = test_dataset.create_dataloader( batch_size=batch_size) batch_X, batch_seq_lens, batch_y = next(iter(train_dataloader)) print ("Sample batch:\n" f" X: {list(batch_X.size())}\n" f" seq_lens: {list(batch_seq_lens.size())}\n" f" y: {list(batch_y.size())}\n" "Sample point:\n" f" X: {batch_X[0]}\n" f" seq_len: {batch_seq_lens[0]}\n" f" y: {batch_y[0]}")  Sample batch: X: [64, 14] seq_lens: [64] y: [64] Sample point: X: tensor([ 16, 1491, 285, 142, 114, 24, 0, 0, 0, 0, 0, 0, 0, 0]) seq_len: 6 y: 3  ### Trainer Let's create the Trainer class that we'll use to facilitate training for our experiments.   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 class Trainer(object): def __init__(self, model, device, loss_fn=None, optimizer=None, scheduler=None): # Set params self.model = model self.device = device self.loss_fn = loss_fn self.optimizer = optimizer self.scheduler = scheduler def train_step(self, dataloader): """Train step.""" # Set model to train mode self.model.train() loss = 0.0 # Iterate over train batches for i, batch in enumerate(dataloader): # Step batch = [item.to(self.device) for item in batch] # Set device inputs, targets = batch[:-1], batch[-1] self.optimizer.zero_grad() # Reset gradients z = self.model(inputs) # Forward pass J = self.loss_fn(z, targets) # Define loss J.backward() # Backward pass self.optimizer.step() # Update weights # Cumulative Metrics loss += (J.detach().item() - loss) / (i + 1) return loss def eval_step(self, dataloader): """Validation or test step.""" # Set model to eval mode self.model.eval() loss = 0.0 y_trues, y_probs = [], [] # Iterate over val batches with torch.inference_mode(): for i, batch in enumerate(dataloader): # Step batch = [item.to(self.device) for item in batch] # Set device inputs, y_true = batch[:-1], batch[-1] z = self.model(inputs) # Forward pass J = self.loss_fn(z, y_true).item() # Cumulative Metrics loss += (J - loss) / (i + 1) # Store outputs y_prob = torch.sigmoid(z).cpu().numpy() y_probs.extend(y_prob) y_trues.extend(y_true.cpu().numpy()) return loss, np.vstack(y_trues), np.vstack(y_probs) def predict_step(self, dataloader): """Prediction step.""" # Set model to eval mode self.model.eval() y_probs = [] # Iterate over val batches with torch.inference_mode(): for i, batch in enumerate(dataloader): # Forward pass w/ inputs inputs, targets = batch[:-1], batch[-1] y_prob = F.softmax(model(inputs), dim=1) # Store outputs y_probs.extend(y_prob) return np.vstack(y_probs) def train(self, num_epochs, patience, train_dataloader, val_dataloader): best_val_loss = np.inf for epoch in range(num_epochs): # Steps train_loss = self.train_step(dataloader=train_dataloader) val_loss, _, _ = self.eval_step(dataloader=val_dataloader) self.scheduler.step(val_loss) # Early stopping if val_loss < best_val_loss: best_val_loss = val_loss best_model = self.model _patience = patience # reset _patience else: _patience -= 1 if not _patience: # 0 print("Stopping early!") break # Logging print( f"Epoch: {epoch+1} | " f"train_loss: {train_loss:.5f}, " f"val_loss: {val_loss:.5f}, " f"lr: {self.optimizer.param_groups[0]['lr']:.2E}, " f"_patience: {_patience}" ) return best_model  ## Attention Attention applied to the outputs from an RNN. In theory, the outputs can come from anywhere where we want to learn how to weight amongst them but since we're working with the context of an RNN from the previous lesson , we'll continue with that. $\alpha = softmax(W_{attn}h)$ $c_t = \sum_{i=1}^{n} \alpha_{t,i}h_i$ Variable Description $$N$$ batch size $$M$$ max sequence length in the batch $$H$$ hidden dim, model dim, etc. $$h$$ RNN outputs (or any group of outputs you want to attend to) $$\in \mathbb{R}^{NXMXH}$$ $$\alpha_{t,i}$$ alignment function context vector $$c_t$$ (attention in our case)
$$W_{attn}$$ attention weights to learn $$\in \mathbb{R}^{HX1}$$
$$c_t$$ context vector that accounts for the different inputs with attention

 1 import torch.nn.functional as F 

The RNN will create an encoded representation for each word in our input resulting in a stacked vector that has dimensions $$NXMXH$$, where N is the # of samples in the batch, M is the max sequence length in the batch, and H is the number of hidden units in the RNN.

 1 2 3 4 BATCH_SIZE = 64 SEQ_LEN = 8 EMBEDDING_DIM = 100 RNN_HIDDEN_DIM = 128 
 1 2 # Embed x = torch.rand((BATCH_SIZE, SEQ_LEN, EMBEDDING_DIM)) 
 1 2 3 4 5 # Encode rnn = nn.RNN(EMBEDDING_DIM, RNN_HIDDEN_DIM, batch_first=True) out, h_n = rnn(x) # h_n is the last hidden state print ("out: ", out.shape) print ("h_n: ", h_n.shape) 
out:  torch.Size([64, 8, 128])
h_n:  torch.Size([1, 64, 128])

  1 2 3 4 5 6 7 8 9 10 # Attend attn = nn.Linear(RNN_HIDDEN_DIM, 1) e = attn(out) attn_vals = F.softmax(e.squeeze(2), dim=1) c = torch.bmm(attn_vals.unsqueeze(1), out).squeeze(1) print ("e: ", e.shape) print ("attn_vals: ", attn_vals.shape) print ("attn_vals[0]: ", attn_vals[0]) print ("sum(attn_vals[0]): ", sum(attn_vals[0])) print ("c: ", c.shape) 
e:  torch.Size([64, 8, 1])
attn_vals:  torch.Size([64, 8])
attn_vals[0]:  tensor([0.1131, 0.1161, 0.1438, 0.1181, 0.1244, 0.1234, 0.1351, 0.1261],
c:  torch.Size([64, 128])

 1 2 3 4 # Predict fc1 = nn.Linear(RNN_HIDDEN_DIM, NUM_CLASSES) output = F.softmax(fc1(c), dim=1) print ("output: ", output.shape) 
output:  torch.Size([64, 4])


In a many-to-many task such as machine translation, our attentional interface will also account for the encoded representation of token in the output as well (via concatenation) so we can know which encoded inputs to attend to based on the encoded output we're focusing on. For more on this, be sure to explore Bahdanau's attention paper.

### Model

Now let's create our RNN based model but with the addition of the attention layer on top of the RNN's outputs.

 1 2 3 RNN_HIDDEN_DIM = 128 DROPOUT_P = 0.1 HIDDEN_DIM = 100 
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 class RNN(nn.Module): def __init__(self, embedding_dim, vocab_size, rnn_hidden_dim, hidden_dim, dropout_p, num_classes, padding_idx=0): super(RNN, self).__init__() # Initialize embeddings self.embeddings = nn.Embedding( embedding_dim=embedding_dim, num_embeddings=vocab_size, padding_idx=padding_idx) # RNN self.rnn = nn.RNN(embedding_dim, rnn_hidden_dim, batch_first=True) # Attention self.attn = nn.Linear(rnn_hidden_dim, 1) # FC weights self.dropout = nn.Dropout(dropout_p) self.fc1 = nn.Linear(rnn_hidden_dim, hidden_dim) self.fc2 = nn.Linear(hidden_dim, num_classes) def forward(self, inputs): # Embed x_in, seq_lens = inputs x_in = self.embeddings(x_in) # Encode out, h_n = self.rnn(x_in) # Attend e = self.attn(out) attn_vals = F.softmax(e.squeeze(2), dim=1) c = torch.bmm(attn_vals.unsqueeze(1), out).squeeze(1) # Predict z = self.fc1(c) z = self.dropout(z) z = self.fc2(z) return z 
 1 2 3 4 5 6 7 # Simple RNN cell model = RNN( embedding_dim=EMBEDDING_DIM, vocab_size=VOCAB_SIZE, rnn_hidden_dim=RNN_HIDDEN_DIM, hidden_dim=HIDDEN_DIM, dropout_p=DROPOUT_P, num_classes=NUM_CLASSES) model = model.to(device) # set device print (model.named_parameters) 
<bound method Module.named_parameters of RNN(
(rnn): RNN(100, 128, batch_first=True)
(attn): Linear(in_features=128, out_features=1, bias=True)
(dropout): Dropout(p=0.1, inplace=False)
(fc1): Linear(in_features=128, out_features=100, bias=True)
(fc2): Linear(in_features=100, out_features=4, bias=True)
)>


### Training

 1 from torch.optim import Adam 
 1 2 3 4 NUM_LAYERS = 1 LEARNING_RATE = 1e-4 PATIENCE = 10 NUM_EPOCHS = 50 
 1 2 3 # Define Loss class_weights_tensor = torch.Tensor(list(class_weights.values())).to(device) loss_fn = nn.CrossEntropyLoss(weight=class_weights_tensor) 
 1 2 3 4 # Define optimizer & scheduler optimizer = Adam(model.parameters(), lr=LEARNING_RATE) scheduler = torch.optim.lr_scheduler.ReduceLROnPlateau( optimizer, mode="min", factor=0.1, patience=3) 
 1 2 3 4 # Trainer module trainer = Trainer( model=model, device=device, loss_fn=loss_fn, optimizer=optimizer, scheduler=scheduler) 
 1 2 3 # Train best_model = trainer.train( NUM_EPOCHS, PATIENCE, train_dataloader, val_dataloader) 

Epoch: 1 | train_loss: 1.21680, val_loss: 1.08622, lr: 1.00E-04, _patience: 10
Epoch: 2 | train_loss: 1.00379, val_loss: 0.93546, lr: 1.00E-04, _patience: 10
Epoch: 3 | train_loss: 0.87091, val_loss: 0.83399, lr: 1.00E-04, _patience: 10
...
Epoch: 48 | train_loss: 0.35045, val_loss: 0.54718, lr: 1.00E-08, _patience: 10
Epoch: 49 | train_loss: 0.35055, val_loss: 0.54718, lr: 1.00E-08, _patience: 10
Epoch: 50 | train_loss: 0.35086, val_loss: 0.54717, lr: 1.00E-08, _patience: 10
Stopping early!


### Evaluation

 1 2 import json from sklearn.metrics import precision_recall_fscore_support 
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 def get_metrics(y_true, y_pred, classes): """Per-class performance metrics.""" # Performance performance = {"overall": {}, "class": {}} # Overall performance metrics = precision_recall_fscore_support(y_true, y_pred, average="weighted") performance["overall"]["precision"] = metrics[0] performance["overall"]["recall"] = metrics[1] performance["overall"]["f1"] = metrics[2] performance["overall"]["num_samples"] = np.float64(len(y_true)) # Per-class performance metrics = precision_recall_fscore_support(y_true, y_pred, average=None) for i in range(len(classes)): performance["class"][classes[i]] = { "precision": metrics[0][i], "recall": metrics[1][i], "f1": metrics[2][i], "num_samples": np.float64(metrics[3][i]), } return performance 
 1 2 3 # Get predictions test_loss, y_true, y_prob = trainer.eval_step(dataloader=test_dataloader) y_pred = np.argmax(y_prob, axis=1) 
 1 2 3 4 # Determine performance performance = get_metrics( y_true=y_test, y_pred=y_pred, classes=label_encoder.classes) print (json.dumps(performance["overall"], indent=2)) 

{
"precision": 0.8133385428975775,
"recall": 0.8137222222222222,
"f1": 0.8133454847232977,
"num_samples": 18000.0
}


### Inference

 1 2 3 4 5 6 7 8 def get_probability_distribution(y_prob, classes): """Create a dict of class probabilities from an array.""" results = {} for i, class_ in enumerate(classes): results[class_] = np.float64(y_prob[i]) sorted_results = {k: v for k, v in sorted( results.items(), key=lambda item: item[1], reverse=True)} return sorted_results 
  1 2 3 4 5 6 7 8 9 10 # Load artifacts device = torch.device("cpu") label_encoder = LabelEncoder.load(fp=Path(dir, "label_encoder.json")) tokenizer = Tokenizer.load(fp=Path(dir, 'tokenizer.json')) model = GRU( embedding_dim=EMBEDDING_DIM, vocab_size=VOCAB_SIZE, rnn_hidden_dim=RNN_HIDDEN_DIM, hidden_dim=HIDDEN_DIM, dropout_p=DROPOUT_P, num_classes=NUM_CLASSES) model.load_state_dict(torch.load(Path(dir, "model.pt"), map_location=device)) model.to(device) 
RNN(
(rnn): RNN(100, 128, batch_first=True)
(attn): Linear(in_features=128, out_features=1, bias=True)
(dropout): Dropout(p=0.1, inplace=False)
(fc1): Linear(in_features=128, out_features=100, bias=True)
(fc2): Linear(in_features=100, out_features=4, bias=True)
)

 1 2 # Initialize trainer trainer = Trainer(model=model, device=device) 
 1 2 3 4 5 6 7 # Dataloader text = "The final tennis tournament starts next week." X = tokenizer.texts_to_sequences([preprocess(text)]) print (tokenizer.sequences_to_texts(X)) y_filler = label_encoder.encode([label_encoder.classes[0]]*len(X)) dataset = Dataset(X=X, y=y_filler) dataloader = dataset.create_dataloader(batch_size=batch_size) 
['final tennis tournament starts next week']

 1 2 3 4 # Inference y_prob = trainer.predict_step(dataloader) y_pred = np.argmax(y_prob, axis=1) label_encoder.decode(y_pred) 
['Sports']

 1 2 3 # Class distributions prob_dist = get_probability_distribution(y_prob=y_prob[0], classes=label_encoder.classes) print (json.dumps(prob_dist, indent=2)) 
{
"Sports": 0.9651875495910645,
"World": 0.03468644618988037,
"Sci/Tech": 8.490968320984393e-05,
}


## Interpretability

Let's use the attention values to see which encoded tokens were most useful in predicting the appropriate label.

 1 2 import collections import seaborn as sns 
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 class InterpretAttn(nn.Module): def __init__(self, embedding_dim, vocab_size, rnn_hidden_dim, hidden_dim, dropout_p, num_classes, padding_idx=0): super(InterpretAttn, self).__init__() # Initialize embeddings self.embeddings = nn.Embedding( embedding_dim=embedding_dim, num_embeddings=vocab_size, padding_idx=padding_idx) # RNN self.rnn = nn.RNN(embedding_dim, rnn_hidden_dim, batch_first=True) # Attention self.attn = nn.Linear(rnn_hidden_dim, 1) # FC weights self.dropout = nn.Dropout(dropout_p) self.fc1 = nn.Linear(rnn_hidden_dim, hidden_dim) self.fc2 = nn.Linear(hidden_dim, num_classes) def forward(self, inputs): # Embed x_in, seq_lens = inputs x_in = self.embeddings(x_in) # Encode out, h_n = self.rnn(x_in) # Attend e = self.attn(out) # could add optional activation function (ex. tanh) attn_vals = F.softmax(e.squeeze(2), dim=1) return attn_vals 
 1 2 3 4 5 6 7 # Initialize model interpretable_model = InterpretAttn( embedding_dim=EMBEDDING_DIM, vocab_size=VOCAB_SIZE, rnn_hidden_dim=RNN_HIDDEN_DIM, hidden_dim=HIDDEN_DIM, dropout_p=DROPOUT_P, num_classes=NUM_CLASSES) interpretable_model.load_state_dict(torch.load(Path(dir, "model.pt"), map_location=device)) interpretable_model.to(device) 
InterpretAttn(
(rnn): RNN(100, 128, batch_first=True)
(attn): Linear(in_features=128, out_features=1, bias=True)
(dropout): Dropout(p=0.1, inplace=False)
(fc1): Linear(in_features=128, out_features=100, bias=True)
(fc2): Linear(in_features=100, out_features=4, bias=True)
)

 1 2 # Initialize trainer interpretable_trainer = Trainer(model=interpretable_model, device=device) 
 1 2 3 # Get attention values attn_vals = interpretable_trainer.predict_step(dataloader) print (attn_vals.shape) # (N, max_seq_len) 
 1 2 3 4 # Visualize a bi-gram filter's outputs sns.set(rc={"figure.figsize":(10, 1)}) tokens = tokenizer.sequences_to_texts(X)[0].split(" ") sns.heatmap(attn_vals, xticklabels=tokens) 

The word tennis was attended to the most to result in the Sports label.

## Types of attention

We'll briefly look at the different types of attention and when to use each them.

### Soft (global) attention

Soft attention the type of attention we've implemented so far, where we attend to all encoded inputs when creating our context vector.

• advantages: we always have the ability to attend to all inputs in case something we saw much earlier/ see later are crucial for determining the output.
• disadvantages: if our input sequence is very long, this can lead to expensive compute.

### Hard attention

Hard attention is focusing on a specific set of the encoded inputs at each time step.

• advantages: we can save a lot of compute on long sequences by only focusing on a local patch each time.
• disadvantages: non-differentiable and so we need to use more complex techniques (variance reduction, reinforcement learning, etc.) to train.

### Local attention

Local attention blends the advantages of soft and hard attention. It involves learning an aligned position vector and empirically determining a local window of encoded inputs to attend to.

• advantages: apply attention to a local patch of inputs yet remain differentiable.
• disadvantages: need to determine the alignment vector for each output but it's a worthwhile trade off to determine the right window of inputs to attend to in order to avoid attending to all of them.

### Self-attention

We can also use attention within the encoded input sequence to create a weighted representation that based on the similarity between input pairs. This will allow us to create rich representations of the input sequence that are aware of the relationships between each other. For example, in the image below you can see that when composing the representation of the token "its", this specific attention head will be incorporating signal from the token "Law" (it's learned that "its" is referring to the "Law").

In the next lesson, we'll implement Transformers that leverage self-attention to create contextual representations of our inputs for downstream applications.

To cite this content, please use:

 1 2 3 4 5 6 @article{madewithml, author = {Goku Mohandas}, title = { Attention - Made With ML }, howpublished = {\url{https://madewithml.com/}}, year = {2023} }