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# Copyright 2022 MosaicML Examples authors
# SPDX-License-Identifier: Apache-2.0
# Adapted from https://github.com/HazyResearch/flash-attention/blob/main/flash_attn/bert_padding.py
# Which was adapted from https://github.com/mlcommons/training_results_v1.1/blob/main/NVIDIA/benchmarks/bert/implementations/pytorch/padding.py
"""Helper functions for padding and unpadding batches. """
from typing import Tuple, cast
import torch
import torch.nn.functional as F
from einops import rearrange, repeat
class IndexFirstAxis(torch.autograd.Function):
@staticmethod
def forward(ctx, input: torch.Tensor, indices: torch.Tensor) -> torch.Tensor:
"""Get just the values of `input` which are at `indices`.
Arguments:
ctx: the autograd context object
input: (b, ...) 2+ dimensional tensor
indices: (num_idx) 1D tensor
"""
ctx.save_for_backward(indices)
assert input.ndim >= 2
ctx.first_axis_dim, other_shape = (
input.shape[0],
input.shape[1:],
) # type: ignore
second_dim = other_shape.numel() # product of sizes of all but first dimension
# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
return torch.gather(
rearrange(input, 'b ... -> b (...)'), # (b, ...) -> (b, second_dim)
0,
repeat(
indices, 'z -> z d', d=second_dim
), # (indices,) -> (indices, second_dim)
).reshape(
-1, *other_shape
) # (num_idx, ...)
@staticmethod
def backward(ctx, grad_output: torch.Tensor) -> Tuple[torch.Tensor, None]:
(indices,) = ctx.saved_tensors
assert grad_output.ndim >= 2
other_shape = grad_output.shape[1:]
grad_output = rearrange(grad_output, 'b ... -> b (...)')
grad_input = torch.zeros(
[ctx.first_axis_dim, grad_output.shape[1]],
device=grad_output.device,
dtype=grad_output.dtype,
)
# TD [2022-03-04] For some reason torch.scatter is a bit faster than indexing.
# grad_input[indices] = grad_output
grad_input.scatter_(
0, repeat(indices, 'z -> z d', d=grad_output.shape[1]), grad_output
)
return grad_input.reshape(ctx.first_axis_dim, *other_shape), None
index_first_axis = IndexFirstAxis.apply
class IndexPutFirstAxis(torch.autograd.Function):
@staticmethod
def forward(
ctx, values: torch.Tensor, indices: torch.Tensor, first_axis_dim
) -> torch.Tensor:
ctx.save_for_backward(indices)
assert indices.ndim == 1
assert values.ndim >= 2
output = torch.zeros(
first_axis_dim, *values.shape[1:], device=values.device, dtype=values.dtype
)
output[indices] = values
return output
@staticmethod
def backward(ctx, grad_output: torch.Tensor) -> Tuple[torch.Tensor, None, None]:
(indices,) = ctx.saved_tensors
grad_values = grad_output[indices]
return grad_values, None, None
index_put_first_axis = IndexPutFirstAxis.apply
def unpad_input(
hidden_states: torch.Tensor,
attention_mask: torch.Tensor,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor, int]:
"""Remove padding from input sequences.
Arguments:
hidden_states: (batch, seqlen, ...)
attention_mask: (batch, seqlen), bool / int, 1 means valid and 0 means not valid.
Returns:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
indices: (total_nnz)
cu_seqlens: (batch + 1), the cumulative sequence lengths, used to index into hidden_states.
max_seqlen_in_batch: int ()
"""
seqlens_in_batch = attention_mask.sum(dim=-1, dtype=torch.int32)
indices = torch.nonzero(attention_mask.flatten(), as_tuple=False).flatten()
max_seqlen_in_batch = int(seqlens_in_batch.max().item())
cu_seqlens = F.pad(torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.int32), (1, 0))
# TD [2022-03-04] We don't want to index with a bool mask, because Pytorch will expand the
# bool mask, then call nonzero to get the indices, then index with those. The indices is @dim
# times larger than it needs to be, wasting memory. It's faster and more memory-efficient to
# index with integer indices. Moreover, torch's index is a bit slower than it needs to be,
# so we write custom forward and backward to make it a bit faster.
hidden_states = cast(
torch.Tensor,
index_first_axis(rearrange(hidden_states, 'b s ... -> (b s) ...'), indices),
)
return hidden_states, indices, cu_seqlens, max_seqlen_in_batch
def unpad_input_only(
hidden_states: torch.Tensor,
attention_mask: torch.Tensor,
) -> torch.Tensor:
"""Like unpad_input, but only return the unpadded first tensor.
Save a small amount of overhead.
Arguments:
hidden_states: (batch, seqlen, ...)
attention_mask: (batch, seqlen), bool / int, 1 means valid and 0 means not valid.
Returns:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
"""
indices = torch.nonzero(attention_mask.flatten(), as_tuple=False).flatten()
return index_first_axis(rearrange(hidden_states, 'b s ... -> (b s) ...'), indices)
def pad_input(
hidden_states: torch.Tensor, indices: torch.Tensor, batch: int, seqlen: int
) -> torch.Tensor:
"""Add padding to sequences.
Arguments:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
indices: (total_nnz)
batch: int batch_size
seqlen: int max sequence length
Returns:
hidden_states: (batch, seqlen, ...)
"""
output = index_put_first_axis(hidden_states, indices, batch * seqlen)
return rearrange(output, '(b s) ... -> b s ...', b=batch)
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