semantic_similarity_with_bert.ipynb

Kernel: Python 3

Semantic Similarity with BERT

Author: Mohamad Merchant
Date created: 2020/08/15
Last modified: 2020/08/29
Description: Natural Language Inference by fine-tuning BERT model on SNLI Corpus.

Introduction

Semantic Similarity is the task of determining how similar two sentences are, in terms of what they mean. This example demonstrates the use of SNLI (Stanford Natural Language Inference) Corpus to predict sentence semantic similarity with Transformers. We will fine-tune a BERT model that takes two sentences as inputs and that outputs a similarity score for these two sentences.

References

• BERT

• SNLI

Setup

Note: install HuggingFace transformers via pip install transformers (version >= 2.11.0).

import numpy as np
import pandas as pd
import tensorflow as tf
import transformers

Configuration

max_length = 128  # Maximum length of input sentence to the model.
batch_size = 32
epochs = 2

# Labels in our dataset.
labels = ["contradiction", "entailment", "neutral"]

Load the Data

!curl -LO https://raw.githubusercontent.com/MohamadMerchant/SNLI/master/data.tar.gz
!tar -xvzf data.tar.gz

# There are more than 550k samples in total; we will use 100k for this example.
train_df = pd.read_csv("SNLI_Corpus/snli_1.0_train.csv", nrows=100000)
valid_df = pd.read_csv("SNLI_Corpus/snli_1.0_dev.csv")
test_df = pd.read_csv("SNLI_Corpus/snli_1.0_test.csv")

# Shape of the data
print(f"Total train samples : {train_df.shape[0]}")
print(f"Total validation samples: {valid_df.shape[0]}")
print(f"Total test samples: {valid_df.shape[0]}")

Dataset Overview:

• sentence1: The premise caption that was supplied to the author of the pair.

• sentence2: The hypothesis caption that was written by the author of the pair.

• similarity: This is the label chosen by the majority of annotators. Where no majority exists, the label "-" is used (we will skip such samples here).

Here are the "similarity" label values in our dataset:

• Contradiction: The sentences share no similarity.

• Entailment: The sentences have similar meaning.

• Neutral: The sentences are neutral.

Let's look at one sample from the dataset:

print(f"Sentence1: {train_df.loc[1, 'sentence1']}")
print(f"Sentence2: {train_df.loc[1, 'sentence2']}")
print(f"Similarity: {train_df.loc[1, 'similarity']}")

Preprocessing

# We have some NaN entries in our train data, we will simply drop them.
print("Number of missing values")
print(train_df.isnull().sum())
train_df.dropna(axis=0, inplace=True)

Distribution of our training targets.

print("Train Target Distribution")
print(train_df.similarity.value_counts())

Distribution of our validation targets.

print("Validation Target Distribution")
print(valid_df.similarity.value_counts())

The value "-" appears as part of our training and validation targets. We will skip these samples.

train_df = (
    train_df[train_df.similarity != "-"]
    .sample(frac=1.0, random_state=42)
    .reset_index(drop=True)
)
valid_df = (
    valid_df[valid_df.similarity != "-"]
    .sample(frac=1.0, random_state=42)
    .reset_index(drop=True)
)

One-hot encode training, validation, and test labels.

train_df["label"] = train_df["similarity"].apply(
    lambda x: 0 if x == "contradiction" else 1 if x == "entailment" else 2
)
y_train = tf.keras.utils.to_categorical(train_df.label, num_classes=3)

valid_df["label"] = valid_df["similarity"].apply(
    lambda x: 0 if x == "contradiction" else 1 if x == "entailment" else 2
)
y_val = tf.keras.utils.to_categorical(valid_df.label, num_classes=3)

test_df["label"] = test_df["similarity"].apply(
    lambda x: 0 if x == "contradiction" else 1 if x == "entailment" else 2
)
y_test = tf.keras.utils.to_categorical(test_df.label, num_classes=3)

Keras Custom Data Generator

class BertSemanticDataGenerator(tf.keras.utils.Sequence):
    """Generates batches of data.

    Args:
        sentence_pairs: Array of premise and hypothesis input sentences.
        labels: Array of labels.
        batch_size: Integer batch size.
        shuffle: boolean, whether to shuffle the data.
        include_targets: boolean, whether to include the labels.

    Returns:
        Tuples `([input_ids, attention_mask, `token_type_ids], labels)`
        (or just `[input_ids, attention_mask, `token_type_ids]`
         if `include_targets=False`)
    """

    def __init__(
        self,
        sentence_pairs,
        labels,
        batch_size=batch_size,
        shuffle=True,
        include_targets=True,
    ):
        self.sentence_pairs = sentence_pairs
        self.labels = labels
        self.shuffle = shuffle
        self.batch_size = batch_size
        self.include_targets = include_targets
        # Load our BERT Tokenizer to encode the text.
        # We will use base-base-uncased pretrained model.
        self.tokenizer = transformers.BertTokenizer.from_pretrained(
            "bert-base-uncased", do_lower_case=True
        )
        self.indexes = np.arange(len(self.sentence_pairs))
        self.on_epoch_end()

    def __len__(self):
        # Denotes the number of batches per epoch.
        return len(self.sentence_pairs) // self.batch_size

    def __getitem__(self, idx):
        # Retrieves the batch of index.
        indexes = self.indexes[idx * self.batch_size : (idx + 1) * self.batch_size]
        sentence_pairs = self.sentence_pairs[indexes]

        # With BERT tokenizer's batch_encode_plus batch of both the sentences are
        # encoded together and separated by [SEP] token.
        encoded = self.tokenizer.batch_encode_plus(
            sentence_pairs.tolist(),
            add_special_tokens=True,
            max_length=max_length,
            return_attention_mask=True,
            return_token_type_ids=True,
            pad_to_max_length=True,
            return_tensors="tf",
        )

        # Convert batch of encoded features to numpy array.
        input_ids = np.array(encoded["input_ids"], dtype="int32")
        attention_masks = np.array(encoded["attention_mask"], dtype="int32")
        token_type_ids = np.array(encoded["token_type_ids"], dtype="int32")

        # Set to true if data generator is used for training/validation.
        if self.include_targets:
            labels = np.array(self.labels[indexes], dtype="int32")
            return [input_ids, attention_masks, token_type_ids], labels
        else:
            return [input_ids, attention_masks, token_type_ids]

    def on_epoch_end(self):
        # Shuffle indexes after each epoch if shuffle is set to True.
        if self.shuffle:
            np.random.RandomState(42).shuffle(self.indexes)

Build the model.

# Create the model under a distribution strategy scope.
strategy = tf.distribute.MirroredStrategy()

with strategy.scope():
    # Encoded token ids from BERT tokenizer.
    input_ids = tf.keras.layers.Input(
        shape=(max_length,), dtype=tf.int32, name="input_ids"
    )
    # Attention masks indicates to the model which tokens should be attended to.
    attention_masks = tf.keras.layers.Input(
        shape=(max_length,), dtype=tf.int32, name="attention_masks"
    )
    # Token type ids are binary masks identifying different sequences in the model.
    token_type_ids = tf.keras.layers.Input(
        shape=(max_length,), dtype=tf.int32, name="token_type_ids"
    )
    # Loading pretrained BERT model.
    bert_model = transformers.TFBertModel.from_pretrained("bert-base-uncased")
    # Freeze the BERT model to reuse the pretrained features without modifying them.
    bert_model.trainable = False

    bert_output = bert_model.bert(
        input_ids, attention_mask=attention_masks, token_type_ids=token_type_ids
    )
    sequence_output = bert_output.last_hidden_state
    pooled_output = bert_output.pooler_output
    # Add trainable layers on top of frozen layers to adapt the pretrained features on the new data.
    bi_lstm = tf.keras.layers.Bidirectional(
        tf.keras.layers.LSTM(64, return_sequences=True)
    )(sequence_output)
    # Applying hybrid pooling approach to bi_lstm sequence output.
    avg_pool = tf.keras.layers.GlobalAveragePooling1D()(bi_lstm)
    max_pool = tf.keras.layers.GlobalMaxPooling1D()(bi_lstm)
    concat = tf.keras.layers.concatenate([avg_pool, max_pool])
    dropout = tf.keras.layers.Dropout(0.3)(concat)
    output = tf.keras.layers.Dense(3, activation="softmax")(dropout)
    model = tf.keras.models.Model(
        inputs=[input_ids, attention_masks, token_type_ids], outputs=output
    )

    model.compile(
        optimizer=tf.keras.optimizers.Adam(),
        loss="categorical_crossentropy",
        metrics=["acc"],
    )


print(f"Strategy: {strategy}")
model.summary()

Create train and validation data generators

train_data = BertSemanticDataGenerator(
    train_df[["sentence1", "sentence2"]].values.astype("str"),
    y_train,
    batch_size=batch_size,
    shuffle=True,
)
valid_data = BertSemanticDataGenerator(
    valid_df[["sentence1", "sentence2"]].values.astype("str"),
    y_val,
    batch_size=batch_size,
    shuffle=False,
)

Train the Model

Training is done only for the top layers to perform "feature extraction", which will allow the model to use the representations of the pretrained model.

history = model.fit(
    train_data,
    validation_data=valid_data,
    epochs=epochs,
    use_multiprocessing=True,
    workers=-1,
)

Fine-tuning

This step must only be performed after the feature extraction model has been trained to convergence on the new data.

This is an optional last step where bert_model is unfreezed and retrained with a very low learning rate. This can deliver meaningful improvement by incrementally adapting the pretrained features to the new data.

# Unfreeze the bert_model.
bert_model.trainable = True
# Recompile the model to make the change effective.
model.compile(
    optimizer=tf.keras.optimizers.Adam(1e-5),
    loss="categorical_crossentropy",
    metrics=["accuracy"],
)
model.summary()

Train the entire model end-to-end.

history = model.fit(
    train_data,
    validation_data=valid_data,
    epochs=epochs,
    use_multiprocessing=True,
    workers=-1,
)

Evaluate model on the test set

test_data = BertSemanticDataGenerator(
    test_df[["sentence1", "sentence2"]].values.astype("str"),
    y_test,
    batch_size=batch_size,
    shuffle=False,
)
model.evaluate(test_data, verbose=1)

Inference on custom sentences

def check_similarity(sentence1, sentence2):
    sentence_pairs = np.array([[str(sentence1), str(sentence2)]])
    test_data = BertSemanticDataGenerator(
        sentence_pairs, labels=None, batch_size=1, shuffle=False, include_targets=False,
    )

    proba = model.predict(test_data[0])[0]
    idx = np.argmax(proba)
    proba = f"{proba[idx]: .2f}%"
    pred = labels[idx]
    return pred, proba

Check results on some example sentence pairs.

sentence1 = "Two women are observing something together."
sentence2 = "Two women are standing with their eyes closed."
check_similarity(sentence1, sentence2)

Check results on some example sentence pairs.

sentence1 = "A smiling costumed woman is holding an umbrella"
sentence2 = "A happy woman in a fairy costume holds an umbrella"
check_similarity(sentence1, sentence2)

Check results on some example sentence pairs

sentence1 = "A soccer game with multiple males playing"
sentence2 = "Some men are playing a sport"
check_similarity(sentence1, sentence2)

Example available on HuggingFace

Trained Model	Demo