generation.py

# coding=utf-8
# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Generation utilities."""

import torch
import torch.nn.functional as F

from megatron import get_args, get_tokenizer, mpu
from megatron.utils import get_ltor_masks_and_position_ids
from .communication import (
    copy_from_last_to_first_pipeline_stage,
    broadcast_from_last_pipeline_stage,
    broadcast_from_last_to_first_pipeline_stage)
from .forward_step import ForwardStep
from .sampling import sample


def generate_tokens_probs_and_return_on_first_stage(
        model, tokens, lengths,
        return_output_log_probs=False,
        greedy=False, top_k=0, top_p=0.0,
        temperature=1.0,
        use_eod_token_for_early_termination=True):
    """Main token generation function.
    Arguments:
        model: no interleaving is supported.
        tokens: prompt tokens extended to be of size [b, max-sequence-length]
        lengths: original prompt length, size: [b]
        return_output_log_probs: flag to calculate the log probability of
            the generated tokens. Note that the log probability is the one
            after logits are modifed for sampling.
        greedy, top_k, top_p: greedy, top-k, and top-p sampling parameters.
            Note that these three paramters are exclusive meaning that:
                if greedy = true then we should have top-k=top-p=0.
                if top-k > 0 then we expect greedy=false and top-p=0.
                if top-p > 0 then we check for greedy=false and top-k=0.
        temperature: sampling temperature.
        use_eod_token_for_early_termination: if True, do early termination if
            all the sequences have reached this token.
    Note: Outside of model, other parameters only need to be available on
          rank 0.
    Outputs: Note that is size is adjusted to a lower value than
             max-sequence-length if generation is terminated early.
        tokens: prompt and generated tokens. size: [b, :]
        generated_sequence_lengths: total length (including prompt) of
            the generated sequence. size: [b]
        output_log_probs: log probability of the selected tokens. size: [b, s]
    """

    args = get_args()
    tokenizer = get_tokenizer()

    batch_size = tokens.size(0)
    min_prompt_length = lengths.min().item()
    max_sequence_length = tokens.size(1)
    max_sequence_length = min(max_sequence_length, args.max_position_embeddings)

    # forward step.
    forward_step = ForwardStep(model, batch_size, max_sequence_length)

    # Added termination_id to support the case that we want to terminate the
    # generation once that id is generated.
    if hasattr(args, 'eos_id'):
        termination_id = args.eos_id
    else:
        termination_id = tokenizer.eod

    # ===================
    # Pre-allocate memory
    # ===================

    # Log probability of the sequence (prompt + generated tokens).
    output_log_probs = None
    output_log_probs_size = (batch_size, max_sequence_length - 1)
    # Lengths of generated seuquence including including prompts.
    generated_sequence_lengths = None
    if mpu.is_pipeline_last_stage():
        if return_output_log_probs:
            output_log_probs = torch.empty(output_log_probs_size,
                                           dtype=torch.float32,
                                           device=torch.cuda.current_device())
        generated_sequence_lengths = torch.ones(
            batch_size, dtype=torch.int64,
            device=torch.cuda.current_device()) * max_sequence_length
    # Whether we have reached a termination id.
    is_generation_done = torch.zeros(batch_size, dtype=torch.uint8,
                                     device=torch.cuda.current_device())

    # =============
    # Run infernece
    # =============

    with torch.no_grad():
        attention_mask, position_ids = _build_attention_mask_and_position_ids(
            tokens)
        prev_context_length = 0
        for context_length in range(min_prompt_length, max_sequence_length):

            # Pick the slice that we need to pass through the network.
            tokens2use = tokens[:, prev_context_length:context_length]
            positions2use = position_ids[:, prev_context_length:context_length]
            attention_mask2use = attention_mask[
                ..., prev_context_length:context_length, :context_length]

            # logits will be meanigful only in the last pipeline stage.
            logits = forward_step(tokens2use, positions2use, attention_mask2use)

            if mpu.is_pipeline_last_stage():
                # Always the last stage should have an output.
                assert logits is not None

                # Sample.
                last_token_logits = logits[:, -1, :]
                new_sample, updated_last_token_logits = sample(
                    last_token_logits,
                    greedy=greedy,
                    top_k=top_k,
                    top_p=top_p,
                    temperature=temperature,
                    vocab_size=tokenizer.vocab_size)
                # Now that we have the sample and updated logits,
                # update the main logits and input tokens.
                # If a prompt length is smaller or equal th current context
                # length, it means we have started generating tokens
                started = lengths <= context_length
                # Update the logits
                last_token_logits.masked_scatter_(
                    started.unsqueeze(1), updated_last_token_logits[started])
                # and the tokens.
                tokens[started, context_length] = new_sample[started]

                # Calculate the log probabilities.
                if return_output_log_probs:
                    log_probs = F.log_softmax(logits, dim=2)
                    if return_output_log_probs:
                        # Pick the tokens that we need to get the log
                        # probabilities for. Note that next input token is
                        # the token which we selected in the current logits,
                        # so shift by 1.
                        indices = torch.unsqueeze(
                            tokens[
                                :,
                                (prev_context_length + 1):(context_length + 1)],
                            2)
                        output_log_probs[:,
                                         prev_context_length:context_length] = \
                            torch.gather(log_probs, 2, indices).squeeze(2)

            # Update the tokens on the first stage so the next input to
            # the network is correct.
            copy_from_last_to_first_pipeline_stage(batch_size, torch.int64,
                                                   tokens[:, context_length])

            # Update the context length for the next token generation.
            prev_context_length = context_length

            # Check if all the sequences have hit the termination_id.
            done = None
            if mpu.is_pipeline_last_stage():
                done_token = (new_sample == termination_id).byte() & \
                    started.byte()
                just_finished = (done_token & ~is_generation_done).bool()
                generated_sequence_lengths[just_finished.view(-1)] = \
                    context_length + 1
                is_generation_done = is_generation_done | done_token
                done = torch.all(is_generation_done)
            done = broadcast_from_last_pipeline_stage(1, torch.uint8,
                                                      tensor=done)
            if use_eod_token_for_early_termination and done:
                break

    # ===================================================
    # Update the length of based on max generated length.
    # ===================================================

    tokens = tokens[:, :(context_length + 1)]
    if mpu.is_pipeline_last_stage():
        if return_output_log_probs:
            output_log_probs = output_log_probs[:, :context_length]

    # ======================================
    # Broadcast to the first pipeline stage.
    # ======================================

    generated_sequence_lengths = broadcast_from_last_to_first_pipeline_stage(
        batch_size, torch.int64, generated_sequence_lengths)
    if return_output_log_probs:
        output_log_probs_size = (batch_size, context_length)
        output_log_probs = broadcast_from_last_to_first_pipeline_stage(
            output_log_probs_size, torch.float32, output_log_probs)

    return tokens, generated_sequence_lengths, output_log_probs



def _build_attention_mask_and_position_ids(tokens):
    """Build the attention mask and postition ids for the input tokens."""

    # Since we are not interested in loss-mask and reset attention/position
    # is also False, eod_token is not used so it is safe to set it to None.
    attention_mask, _, position_ids = get_ltor_masks_and_position_ids(
        data=tokens,
        eod_token=None,
        reset_position_ids=False,
        reset_attention_mask=False,
        eod_mask_loss=False)

    return attention_mask, position_ids