mirror of https://github.com/hpcaitech/ColossalAI
76 lines
2.9 KiB
Python
76 lines
2.9 KiB
Python
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from colossalai.context.parallel_mode import ParallelMode
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import torch
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from torch.cuda.amp import custom_bwd, custom_fwd
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class _VocabCrossEntropy(torch.autograd.Function):
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@staticmethod
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@custom_fwd
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def forward(ctx, vocab_parallel_logits, target):
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# Maximum value along vocab dimension across all GPUs.
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logits_max = torch.max(vocab_parallel_logits, dim=-1)[0]
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# Subtract the maximum value.
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vocab_parallel_logits.sub_(logits_max.unsqueeze(dim=-1))
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# Create a mask of valid vocab ids (1 means it needs to be masked).
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target_mask = target < 0
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masked_target = target.clone()
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masked_target[target_mask] = 0
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# Get predicted-logits = logits[target].
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# For Simplicity, we convert logits to a 2-D tensor with size
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# [*, partition-vocab-size] and target to a 1-D tensor of size [*].
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logits_2d = vocab_parallel_logits.view(-1, vocab_parallel_logits.size(-1))
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masked_target_1d = masked_target.view(-1)
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arange_1d = torch.arange(start=0, end=logits_2d.size()[0],
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device=logits_2d.device)
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predicted_logits_1d = logits_2d[arange_1d, masked_target_1d]
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predicted_logits_1d = predicted_logits_1d.clone().contiguous()
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predicted_logits = predicted_logits_1d.view_as(target)
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predicted_logits[target_mask] = 0.0
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# Sum of exponential of logits along vocab dimension across all GPUs.
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exp_logits = vocab_parallel_logits
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torch.exp(vocab_parallel_logits, out=exp_logits)
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sum_exp_logits = exp_logits.sum(dim=-1)
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# Loss = log(sum(exp(logits))) - predicted-logit.
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loss = torch.log(sum_exp_logits) - predicted_logits
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# Store softmax, target-mask and masked-target for backward pass.
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exp_logits.div_(sum_exp_logits.unsqueeze(dim=-1))
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ctx.save_for_backward(exp_logits, target_mask, masked_target_1d)
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return loss
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@staticmethod
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@custom_bwd
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def backward(ctx, grad_output):
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# Retreive tensors from the forward path.
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softmax, target_mask, masked_target_1d = ctx.saved_tensors
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# All the inputs have softmax as their gradient.
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grad_input = softmax
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# For simplicity, work with the 2D gradient.
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partition_vocab_size = softmax.size()[-1]
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grad_2d = grad_input.view(-1, partition_vocab_size)
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# Add the gradient from matching classes.
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arange_1d = torch.arange(start=0, end=grad_2d.size()[0],
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device=grad_2d.device)
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grad_2d[arange_1d, masked_target_1d] -= (
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1.0 - target_mask.view(-1).float())
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# Finally elementwise multiplication with the output gradients.
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grad_input.mul_(grad_output.unsqueeze(dim=-1))
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return grad_input, None
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def vocab_cross_entropy(vocab_logits, target):
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"""helper function for the cross entropy."""
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return _VocabCrossEntropy.apply(vocab_logits, target)
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