generation.py

from typing import List, Optional, Union, Tuple

import torch
import torch.nn.functional as F
from transformers import GPT2LMHeadModel, LogitsProcessorList, LogitsProcessor, PreTrainedTokenizer
from transformers.generation_utils import GenerationMixin, SampleOutput, SampleEncoderDecoderOutput, SampleDecoderOnlyOutput


class SelfDebiasingLogitsProcessor(LogitsProcessor):
    """This class represents a logits processor that applies self-debiasing."""

    def __init__(self, num_debiasing_prefixes: int, decay_constant: float = 100, epsilon: float = 0.01, debug: bool = False,
                 tokenizer: Optional[PreTrainedTokenizer] = None):
        """
        :param num_debiasing_prefixes: the number of debiasing prefixes used
        :param decay_constant: the decay constant (lambda in the paper)
        :param epsilon: the minimum factor by which each probability is multiplied
        :param debug: whether to print additional debugging output
        :param tokenizer: a tokenizer used to print debugging output
        """
        assert not debug or tokenizer, "If debug=True, a tokenizer must be passed to SelfDebiasingLogitsProcessor()"
        self.num_debiasing_prefixes = num_debiasing_prefixes
        self.decay_constant = decay_constant
        self.epsilon = epsilon
        self.debug = debug
        self.tokenizer = tokenizer

    def __call__(self, input_ids: torch.LongTensor, scores: torch.FloatTensor) -> torch.FloatTensor:
        batch_size = scores.shape[0] // (1 + self.num_debiasing_prefixes)
        regular_sentence_indices = range(batch_size)
        for regular_sentence_idx in regular_sentence_indices:
            bias_indices = self._get_bias_indices(regular_sentence_idx, batch_size)
            if bias_indices:
                self._debias_scores(scores, regular_sentence_idx, bias_indices)
        return scores

    def _get_bias_indices(self, regular_sentence_idx: int, batch_size: int) -> List[int]:
        """Returns the indices of all self-debiasing inputs for a regular input"""
        return [regular_sentence_idx + (prefix_idx + 1) * batch_size for prefix_idx in range(self.num_debiasing_prefixes)]

    def _debias_scores(self, scores: torch.FloatTensor, regular_sent_idx: int, bias_indices: List[int]) -> None:
        """Partially debiases the given scores considering a single sentence and the corresponding self-debiasing inputs"""
        logits_biased = [scores[bias_idx] for bias_idx in bias_indices]

        mask = self._generate_decay_mask(scores[regular_sent_idx], logits_biased)
        scores[regular_sent_idx] = torch.log(self._apply_decay_mask(scores[regular_sent_idx], mask))

        for debiasing_sent_idx in bias_indices:
            scores[debiasing_sent_idx] = scores[regular_sent_idx]

    def _apply_decay_mask(self, logits: torch.Tensor, decay_mask: torch.Tensor) -> torch.Tensor:
        """Applies exponential decay to a tensor of logits"""
        probabilities = logits.softmax(dim=-1)
        decay_mask = torch.exp(- decay_mask * self.decay_constant)
        decay_mask = torch.max(decay_mask, torch.tensor([self.epsilon], device=decay_mask.device))
        probabilities = probabilities * decay_mask
        probabilities = probabilities / probabilities.sum(dim=-1)
        return probabilities

    def _generate_decay_mask(self, logits_regular: torch.FloatTensor, logits_biased_list: List[torch.FloatTensor]) -> torch.Tensor:
        """Computes the alpha values (see paper) for each token and stores them in a mask tensor"""
        p_regular = logits_regular.softmax(dim=-1)
        p_biased = None

        for logits_biased in logits_biased_list:
            if p_biased is None:
                p_biased = logits_biased.softmax(dim=-1)
            else:
                p_biased = torch.max(p_biased, logits_biased.softmax(dim=-1))

        if self.debug:
            print(f'== Before Debiasing ==\n'
                  f'Top 5 predictions (regular): {self._get_most_likely_tokens(p_regular, k=5)}\n'
                  f'Top 5 predictions (biased): {self._get_most_likely_tokens(p_biased, k=5)}')

        mask = torch.max(p_biased - p_regular, torch.tensor([0.], device=p_regular.device))

        if self.debug:
            p_regular = self._apply_decay_mask(logits_regular, mask)
            print(f'== After Debiasing ==\n'
                  f'Top 5 predictions (regular): {self._get_most_likely_tokens(p_regular, k=5)}')

        return mask

    def _get_most_likely_tokens(self, probabilities_tensor: torch.Tensor, k: int) -> List[Tuple[str, float]]:
        """Returns the most likely tokens according to a tensor of probabilities"""
        assert len(probabilities_tensor.shape) == 1
        values, indices = torch.topk(probabilities_tensor, k=k, dim=-1)
        tokens = self.tokenizer.convert_ids_to_tokens(indices)
        return list(zip(tokens, [pv.item() for pv in values]))


class SelfDebiasingGPT2LMHeadModel(GPT2LMHeadModel, GenerationMixin):
    """
    This class represents a regular GPT2LMHeadModel that additionally has the capacity to perform self-debiasing. For self-debiasing, the
    init_logits_processor function must be called. Otherwise, this model just performs regular language modeling.
    """

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.logits_processor = None  # type: Optional[SelfDebiasingLogitsProcessor]

    def init_logits_processor(self, *args, **kwargs):
        """Initialize the logits processor. For a list of arguments, see the self-debiasing logit processor's init function."""
        self.logits_processor = SelfDebiasingLogitsProcessor(*args, **kwargs)

    def _get_logits_processor(self, *args, **kwargs) -> LogitsProcessorList:
        logits_processor = super()._get_logits_processor(*args, **kwargs)
        if self.logits_processor is not None:
            logits_processor.append(self.logits_processor)
        return logits_processor

    def beam_sample(self, *args, **kwargs):
        raise NotImplementedError("Beam sampling is not implemented for self-debiasing models")

    def sample(self, input_ids: torch.LongTensor, logits_processor: Optional[LogitsProcessorList] = None,
               logits_warper: Optional[LogitsProcessorList] = None, max_length: Optional[int] = None, pad_token_id: Optional[int] = None,
               eos_token_id: Optional[int] = None, output_attentions: Optional[bool] = None, output_hidden_states: Optional[bool] = None,
               output_scores: Optional[bool] = None, return_dict_in_generate: Optional[bool] = None, **model_kwargs) -> Union[
        SampleOutput, torch.LongTensor]:
        """
        This is a verbatim copy of the original implementation by huggingface, with a single modification to ensure that a text and all
        corresponding self-debiasing inputs always chose the same token to generate next. This modification is enclosed by the texts
        "BEGIN MODIFICATIONS" and "END MODIFICATIONS", respectively.
        """
        # init values
        logits_processor = logits_processor if logits_processor is not None else LogitsProcessorList()
        logits_warper = logits_warper if logits_warper is not None else LogitsProcessorList()
        max_length = max_length if max_length is not None else self.config.max_length
        pad_token_id = pad_token_id if pad_token_id is not None else self.config.pad_token_id
        eos_token_id = eos_token_id if eos_token_id is not None else self.config.eos_token_id
        output_scores = output_scores if output_scores is not None else self.config.output_scores
        output_attentions = output_attentions if output_attentions is not None else self.config.output_attentions
        output_hidden_states = (
            output_hidden_states if output_hidden_states is not None else self.config.output_hidden_states
        )
        return_dict_in_generate = (
            return_dict_in_generate if return_dict_in_generate is not None else self.config.return_dict_in_generate
        )

        # init attention / hidden states / scores tuples
        scores = () if (return_dict_in_generate and output_scores) else None
        decoder_attentions = () if (return_dict_in_generate and output_attentions) else None
        decoder_hidden_states = () if (return_dict_in_generate and output_hidden_states) else None

        # if model is an encoder-decoder, retrieve encoder attention weights and hidden states
        if return_dict_in_generate and self.config.is_encoder_decoder:
            encoder_attentions = model_kwargs["encoder_outputs"].get("attentions") if output_attentions else None
            encoder_hidden_states = (
                model_kwargs["encoder_outputs"].get("hidden_states") if output_hidden_states else None
            )

        # init sequence length tensors
        sequence_lengths, unfinished_sequences, cur_len = self._init_sequence_length_for_generation(
            input_ids, max_length
        )

        # auto-regressive generation
        while cur_len < max_length:
            # prepare model inputs
            model_inputs = self.prepare_inputs_for_generation(input_ids, **model_kwargs)

            # forward pass to get next token
            outputs = self(
                **model_inputs,
                return_dict=True,
                output_attentions=output_attentions,
                output_hidden_states=output_hidden_states,
            )
            next_token_logits = outputs.logits[:, -1, :]

            # pre-process distribution
            next_token_scores = logits_processor(input_ids, next_token_logits)
            next_token_scores = logits_warper(input_ids, next_token_scores)

            # Store scores, attentions and hidden_states when required
            if return_dict_in_generate:
                if output_scores:
                    scores += (next_token_scores,)
                if output_attentions:
                    decoder_attentions += (
                        (outputs.decoder_attentions,) if self.config.is_encoder_decoder else (outputs.attentions,)
                    )

                if output_hidden_states:
                    decoder_hidden_states += (
                        (outputs.decoder_hidden_states,)
                        if self.config.is_encoder_decoder
                        else (outputs.hidden_states,)
                    )

            # sample
            probs = F.softmax(next_token_scores, dim=-1)
            next_tokens = torch.multinomial(probs, num_samples=1).squeeze(1)

            # =========================
            # BEGIN MODIFICATIONS
            # the following modification to the sample method is necessary to ensure that each debiasing sentence is continued in the same
            # way as the original sentence
            if self.logits_processor is not None:
                batch_size = next_tokens.shape[0] // (1 + self.logits_processor.num_debiasing_prefixes)
                regular_sentence_indices = range(batch_size)
                for regular_sentence_idx in regular_sentence_indices:
                    debiasing_sentence_indices = self.logits_processor._get_bias_indices(regular_sentence_idx, batch_size)
                    for debiasing_sentence_idx in debiasing_sentence_indices:
                        next_tokens[debiasing_sentence_idx] = next_tokens[regular_sentence_idx]
            # END MODIFICATIONS
            # =========================

            # add code that transfomers next_tokens to tokens_to_add
            if eos_token_id is not None:
                assert pad_token_id is not None, "If eos_token_id is defined, make sure that pad_token_id is defined."
                next_tokens = next_tokens * unfinished_sequences + (pad_token_id) * (1 - unfinished_sequences)

            # add token and increase length by one
            input_ids = torch.cat([input_ids, next_tokens[:, None]], dim=-1)
            cur_len = cur_len + 1

            # update sequence length
            if eos_token_id is not None:
                sequence_lengths, unfinished_sequences = self._update_seq_length_for_generation(
                    sequence_lengths, unfinished_sequences, cur_len, next_tokens == eos_token_id
                )

            # stop when there is a </s> in each sentence, or if we exceed the maximul length
            if unfinished_sequences.max() == 0:
                break

            # update model kwargs
            model_kwargs = self._update_model_kwargs_for_generation(
                outputs, model_kwargs, is_encoder_decoder=self.config.is_encoder_decoder
            )

        if return_dict_in_generate:
            if self.config.is_encoder_decoder:
                return SampleEncoderDecoderOutput(
                    sequences=input_ids,
                    scores=scores,
                    encoder_attentions=encoder_attentions,
                    encoder_hidden_states=encoder_hidden_states,
                    decoder_attentions=decoder_attentions,
                    decoder_hidden_states=decoder_hidden_states,
                )
            else:
                return SampleDecoderOnlyOutput(
                    sequences=input_ids,
                    scores=scores,
                    attentions=decoder_attentions,
                    hidden_states=decoder_hidden_states,
                )
        else:
            return input_ids