embedding.py

# SPDX-FileCopyrightText: Copyright (c) 2022-2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
#
# 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,
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import math

from ..functional import embedding, unsqueeze, where
from ..module import Module
from ..parameter import Parameter


class Embedding(Module):
    """
    The embedding layer takes input indices (x) and the embedding lookup table (weight) as input.
    And output the corresponding embeddings according to input indices.
    The size of weight is [num_embeddings, embedding_dim]

    Four parameters (tp_size, tp_group, sharding_dim, tp_rank) are involved in tensor parallelism.
    Only when "tp_size > 1 and tp_group is not None", tensor parallelism is enabled.
        When "sharding_dim == 0", the weight is shared in the vocabulary dimension.
            tp_rank must be set when sharding_dim == 0.
        When "sharding_dim == 1",  the weight is shard in the hidden dimension.
    """

    def __init__(self,
                 num_embeddings,
                 embedding_dim,
                 dtype=None,
                 tp_size=1,
                 tp_group=None,
                 sharding_dim=0,
                 tp_rank=None):
        super().__init__()
        # num_embeddings records the total vocab size no matter using TP or not
        self.num_embeddings = num_embeddings
        self.embedding_dim = embedding_dim
        self.tp_size = tp_size
        self.tp_group = tp_group
        self.sharding_dim = sharding_dim
        self.tp_rank = tp_rank

        if sharding_dim == 1:
            self.weight = Parameter(shape=(self.num_embeddings,
                                           self.embedding_dim // self.tp_size),
                                    dtype=dtype)
        elif sharding_dim == 0:
            self.weight = Parameter(shape=(math.ceil(
                self.num_embeddings / self.tp_size), self.embedding_dim),
                                    dtype=dtype)

    def forward(self, x):
        return embedding(x,
                         self.weight.value,
                         tp_size=self.tp_size,
                         tp_group=self.tp_group,
                         sharding_dim=self.sharding_dim,
                         tp_rank=self.tp_rank)


class PromptTuningEmbedding(Embedding):
    """
    PromptTuningEmbedding handles fine-tuned prompts with virtual tokens. At runtime,
    a supplementary embedding dictionary is passed. Tokens whose ids are >= vocab_size are embedded
    with that additional dictionary.
    The prompt tuning dictionary holds multiple tasks, and each sequence is assigned a given task.
    Prompt-tuned tokens from a given sequence use the adequate task dictionary, as defined by the `tasks` input.
    """

    def __init__(self,
                 num_embeddings,
                 embedding_dim,
                 vocab_size=None,
                 dtype=None,
                 tp_size=1,
                 tp_group=None,
                 sharding_dim=0,
                 tp_rank=0):
        super().__init__(num_embeddings, embedding_dim, dtype, tp_size,
                         tp_group, sharding_dim, tp_rank)
        if vocab_size is None:
            vocab_size = num_embeddings
        self.vocab_size = vocab_size

    def forward(self, tokens, prompt_embedding_table, tasks, task_vocab_size):
        """
            Pass all tokens through both normal and prompt embedding tables.
            Tokens are masked so that "normal" embedding only see "normal" tokens. Same logic for "prompt" embedding.
            After those two embedding, combine results based on whether the token was "normal" or "prompt-tuned".

        Parameters:
            tokens : Tensor
                the ids to embbed, size [batch_size, seq_len]

            prompt_embedding_table : Tensor
                the additional embedding table for prompt-tuned tokens, size [num_tasks * num_tokens_per_task, hidden_size]

            tasks: Tensor
                the task required by each token, size [batch_size, seq_len]

            task_vocab_size: Tensor
                the number of tokens used for each task, should be equal to prompt_embedding_table's num_tokens_per_task, size [1]

        Returns:
            Tokens' embedding
        """
        # do not use ">=" because internally the layer works with floating points
        prompt_tokens_mask = tokens > (self.vocab_size - 1)

        # clip tokens in the [0, vocab_size) range
        normal_tokens = where(prompt_tokens_mask, self.vocab_size - 1, tokens)
        normal_embeddings = embedding(normal_tokens, self.weight.value,
                                      self.tp_size, self.tp_group,
                                      self.sharding_dim, self.tp_rank)

        # put virtual tokens in the [0, max_prompt_vocab_size) range
        prompt_tokens = where(prompt_tokens_mask, tokens - self.vocab_size, 0)

        # add offsets to match the concatenated embedding tables
        tasks = tasks * task_vocab_size

        # tasks: [batch_size, seq_len]
        # prompt_tokens: [batch_size, seq_len]
        prompt_tokens = prompt_tokens + tasks
        prompt_embeddings = embedding(prompt_tokens, prompt_embedding_table)

        # prompt_tokens_mask: [batch_size, seq_len] -> [batch_size, seq_len, 1]
        # combine the correct sources of embedding: normal/prompt
        return where(unsqueeze(prompt_tokens_mask, -1), prompt_embeddings,
                     normal_embeddings)