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# This implementation was adapted from https://github.com/Dao-AILab/flash-attention/blob/main/flash_attn/modules/block.py
# Commit id: c94cd09744d20f0ac587a351ff6ff2e8ad11ae1b
# Previously adapted from https://github.com/mlcommons/training_results_v1.1/blob/main/NVIDIA/benchmarks/bert/implementations/pytorch/padding.py
import torch
import torch.nn.functional as F
from einops import rearrange, repeat
class IndexFirstAxis(torch.autograd.Function):
@staticmethod
def forward(ctx, input, indices):
ctx.save_for_backward(indices)
assert input.ndim >= 2
ctx.first_axis_dim, other_shape = input.shape[0], input.shape[1:]
second_dim = other_shape.numel()
# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
# return input[indices]
return torch.gather(
rearrange(input, "b ... -> b (...)"),
0,
repeat(indices, "z -> z d", d=second_dim),
).reshape(-1, *other_shape)
@staticmethod
def backward(ctx, grad_output):
(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, indices, first_axis_dim):
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
)
# TD [2022-03-04] For some reason torch.scatter is a bit faster than indexing.
output[indices] = values
# output.scatter_(0, repeat(indices, 'z -> z d', d=values.shape[1]), values)
return output
@staticmethod
def backward(ctx, grad_output):
(indices,) = ctx.saved_tensors
# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
grad_values = grad_output[indices]
# grad_values = torch.gather(grad_output, 0, repeat(indices, 'z -> z d', d=grad_output.shape[1]))
return grad_values, None, None
index_put_first_axis = IndexPutFirstAxis.apply
class IndexFirstAxisResidual(torch.autograd.Function):
@staticmethod
def forward(ctx, input, indices):
ctx.save_for_backward(indices)
assert input.ndim >= 2
ctx.first_axis_dim, other_shape = input.shape[0], input.shape[1:]
second_dim = other_shape.numel()
# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
output = input[indices]
# We don't want to reshape input (b ... -> b (...)) since it could change the channel_last
# memory format to channel_first. In other words, input might not be contiguous.
# If we don't detach, Pytorch complains about output being a view and is being modified inplace
return output, input.detach()
@staticmethod
def backward(ctx, grad_output, grad_residual):
(indices,) = ctx.saved_tensors
assert grad_output.ndim >= 2
other_shape = grad_output.shape[1:]
assert grad_residual.shape[1:] == other_shape
grad_input = grad_residual
# grad_input[indices] += grad_output
indices = indices.reshape(indices.shape[0], *((1,) * (grad_output.ndim - 1)))
indices = indices.expand_as(grad_output)
grad_input.scatter_add_(0, indices, grad_output)
return grad_input.reshape(ctx.first_axis_dim, *other_shape), None
index_first_axis_residual = IndexFirstAxisResidual.apply
def unpad_input(hidden_states, attention_mask, adapter_mask=None):
"""
Arguments:
hidden_states: (batch, seqlen, ...)
attention_mask: (batch, seqlen), bool / int, 1 means valid and 0 means not valid.
Return:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
indices: (total_nnz), the indices of non-masked tokens from the flattened input sequence.
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 = seqlens_in_batch.max().item()
cu_seqlens = F.pad(
torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.torch.int32), (1, 0)
)
cu_adapter_mask = (
torch.repeat_interleave(adapter_mask, cu_seqlens[1:] - cu_seqlens[:-1])
if adapter_mask is not None
else None
)
# 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.
return (
index_first_axis(rearrange(hidden_states, "b s ... -> (b s) ..."), indices),
indices,
cu_seqlens,
max_seqlen_in_batch,
cu_adapter_mask,
)
def unpad_input_for_concatenated_sequences(hidden_states, attention_mask_in_length):
"""
Supports concatenating short samples in one sequence. The attention_mask_in_length is utilized to mask other short samples. It helps efficient training of variant lengths-based samples (e.g., the supervised fine-tuning task in large language model).
The motivation for this function is explained [here](https://github.com/Dao-AILab/flash-attention/issues/432#issuecomment-1668822286).
For example, if batch = 3 and seqlen = 6, the attention_mask_in_length is:
```
[
[2, 3, 0, 0, 0, 0],
[3, 2, 0, 0, 0, 0],
[6, 0, 0, 0, 0, 0]
]
```
, which refers to the 3D-attention mask:
```
[
[
[1, 0, 0, 0, 0, 0],
[1, 1, 0, 0, 0, 0],
[0, 0, 1, 0, 0, 0],
[0, 0, 1, 1, 0, 0],
[0, 0, 1, 1, 1, 0],
[0, 0, 0, 0, 0, 1]
],
[
[1, 0, 0, 0, 0, 0],
[1, 1, 0, 0, 0, 0],
[1, 1, 1, 0, 0, 0],
[0, 0, 0, 1, 0, 0],
[0, 0, 0, 1, 1, 0],
[0, 0, 0, 0, 0, 1]
],
[
[1, 0, 0, 0, 0, 0],
[1, 1, 0, 0, 0, 0],
[1, 1, 1, 0, 0, 0],
[1, 1, 1, 1, 0, 0],
[1, 1, 1, 1, 1, 0],
[1, 1, 1, 1, 1, 1]
]
]
```.
Arguments:
hidden_states: (batch, seqlen, ...)
attention_mask_in_length: (batch, seqlen), int, a nonzero number (e.g., 1, 2, 3, etc.) means length of concatenated sequence in b-th batch, and 0 means none.
Return:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
indices: (total_nnz), the indices of non-masked tokens from the flattened input sequence.
cu_seqlens: (batch + 1), the cumulative sequence lengths, used to index into hidden_states.
max_seqlen_in_batch: int
"""
length = attention_mask_in_length.sum(dim=-1)
seqlen = attention_mask_in_length.size(-1)
attention_mask_2d = torch.arange(
seqlen, device=length.device, dtype=length.dtype
).expand(len(length), seqlen) < length.unsqueeze(1)
real_indices_idx = torch.nonzero(
attention_mask_in_length.flatten(), as_tuple=False
).flatten()
seqlens_in_batch = attention_mask_in_length.flatten()[real_indices_idx]
indices = torch.nonzero(attention_mask_2d.flatten(), as_tuple=False).flatten()
max_seqlen_in_batch = seqlens_in_batch.max().item()
cu_seqlens = F.pad(
torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.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.
return (
index_first_axis(rearrange(hidden_states, "b s ... -> (b s) ..."), indices),
indices,
cu_seqlens,
max_seqlen_in_batch,
)
def pad_input(hidden_states, indices, batch, seqlen):
"""
Arguments:
hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
indices: (total_nnz), the indices that represent the non-masked tokens of the original padded input sequence.
batch: int, batch size for the padded sequence.
seqlen: int, maximum sequence length for the padded sequence.
Return:
hidden_states: (batch, seqlen, ...)
"""
dim = hidden_states.shape[-1]
# output = torch.zeros((batch * seqlen), dim, device=hidden_states.device, dtype=hidden_states.dtype)
# output[indices] = hidden_states
output = index_put_first_axis(hidden_states, indices, batch * seqlen)
return rearrange(output, "(b s) ... -> b s ...", b=batch)
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