Using BERT for next sentence prediction

Question

Google's BERT is pretrained on next sentence prediction tasks, but I'm wondering if it's possible to call the next sentence prediction function on new data.

The idea is: given sentence A and given sentence B, I want a probabilistic label for whether or not sentence B follows sentence A. BERT is pretrained on a huge set of data, so I was hoping to use this next sentence prediction on new sentence data. I can't seem to figure out if this next sentence prediction function can be called and if so, how. Thanks for your help!

Answer 1

The answer by Aerin is out-dated. The HuggingFace library (now called transformers) has changed a lot over the last couple of months. Here is an example of how to use the next sentence prediction (NSP) model, and how to extract probabilities from it. NOTE this will only work well if you use a model that has a pretrained head for the NSP task.

from torch.nn.functional import softmax
from transformers import BertForNextSentencePrediction, BertTokenizer


seq_A = 'I like cookies !'
seq_B = 'Do you like them ?'

# load pretrained model and a pretrained tokenizer
model = BertForNextSentencePrediction.from_pretrained('bert-base-cased')
tokenizer = BertTokenizer.from_pretrained('bert-base-cased')

# encode the two sequences. Particularly, make clear that they must be 
# encoded as "one" input to the model by using 'seq_B' as the 'text_pair'
encoded = tokenizer.encode_plus(seq_A, text_pair=seq_B, return_tensors='pt')
print(encoded)
# {'input_ids': tensor([[  101,   146,  1176, 18621,   106,   102,  2091,  1128,  1176,  1172, 136,   102]]),
#  'token_type_ids': tensor([[0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1]]),
#  'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
# NOTE how the token_type_ids are 0 for all tokens in seq_A and 1 for seq_B, 
# this way the model knows which token belongs to which sequence

# a model's output is a tuple, we only need the output tensor containing
# the relationships which is the first item in the tuple
seq_relationship_logits = model(**encoded)[0]

# we still need softmax to convert the logits into probabilities
# index 0: sequence B is a continuation of sequence A
# index 1: sequence B is a random sequence
probs = softmax(seq_relationship_logits, dim=1)

print(seq_relationship_logits)
print(probs)
# tensor([[9.9993e-01, 6.7607e-05]], grad_fn=<SoftmaxBackward>)
# very high value for index 0: high probability of seq_B being a continuation of seq_A

Answer 2

Hugging face did it for you: https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/pytorch_pretrained_bert/modeling.py#L854

class BertForNextSentencePrediction(BertPreTrainedModel):
    """BERT model with next sentence prediction head.
    This module comprises the BERT model followed by the next sentence classification head.
    Params:
        config: a BertConfig class instance with the configuration to build a new model.
    Inputs:
        `input_ids`: a torch.LongTensor of shape [batch_size, sequence_length]
            with the word token indices in the vocabulary(see the tokens preprocessing logic in the scripts
            `extract_features.py`, `run_classifier.py` and `run_squad.py`)
        `token_type_ids`: an optional torch.LongTensor of shape [batch_size, sequence_length] with the token
            types indices selected in [0, 1]. Type 0 corresponds to a `sentence A` and type 1 corresponds to
            a `sentence B` token (see BERT paper for more details).
        `attention_mask`: an optional torch.LongTensor of shape [batch_size, sequence_length] with indices
            selected in [0, 1]. It's a mask to be used if the input sequence length is smaller than the max
            input sequence length in the current batch. It's the mask that we typically use for attention when
            a batch has varying length sentences.
        `next_sentence_label`: next sentence classification loss: torch.LongTensor of shape [batch_size]
            with indices selected in [0, 1].
            0 => next sentence is the continuation, 1 => next sentence is a random sentence.
    Outputs:
        if `next_sentence_label` is not `None`:
            Outputs the total_loss which is the sum of the masked language modeling loss and the next
            sentence classification loss.
        if `next_sentence_label` is `None`:
            Outputs the next sentence classification logits of shape [batch_size, 2].
    Example usage:
    ```python
    # Already been converted into WordPiece token ids
    input_ids = torch.LongTensor([[31, 51, 99], [15, 5, 0]])
    input_mask = torch.LongTensor([[1, 1, 1], [1, 1, 0]])
    token_type_ids = torch.LongTensor([[0, 0, 1], [0, 1, 0]])
    config = BertConfig(vocab_size_or_config_json_file=32000, hidden_size=768,
        num_hidden_layers=12, num_attention_heads=12, intermediate_size=3072)
    model = BertForNextSentencePrediction(config)
    seq_relationship_logits = model(input_ids, token_type_ids, input_mask)
    ```
    """
    def __init__(self, config):
        super(BertForNextSentencePrediction, self).__init__(config)
        self.bert = BertModel(config)
        self.cls = BertOnlyNSPHead(config)
        self.apply(self.init_bert_weights)

    def forward(self, input_ids, token_type_ids=None, attention_mask=None, next_sentence_label=None):
        _, pooled_output = self.bert(input_ids, token_type_ids, attention_mask,
                                     output_all_encoded_layers=False)
        seq_relationship_score = self.cls( pooled_output)

        if next_sentence_label is not None:
            loss_fct = CrossEntropyLoss(ignore_index=-1)
            next_sentence_loss = loss_fct(seq_relationship_score.view(-1, 2), next_sentence_label.view(-1))
            return next_sentence_loss
        else:
            return seq_relationship_score