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import torch
import torch.nn.functional as F
import fmhalib as mha

class FMHAFun(torch.autograd.Function):
    @staticmethod
    def forward(ctx, qkv, cu_seqlens, p_dropout, max_s, is_training):
        batch_size = cu_seqlens.numel() - 1
        if batch_size < 4:
            context, S_dmask = mha.fwd_nl(qkv, cu_seqlens, p_dropout, max_s, is_training, None)
        else:
            context, S_dmask = mha.fwd(qkv, cu_seqlens, p_dropout, max_s, is_training, None)
        ctx.save_for_backward(qkv, S_dmask)
        ctx.cu_seqlens = cu_seqlens
        ctx.p_dropout = p_dropout
        ctx.max_s = max_s
        return context
    
    @staticmethod
    def backward(ctx, dout):
        qkv, S_dmask = ctx.saved_tensors
        batch_size = ctx.cu_seqlens.numel() - 1
        if batch_size < 4:
            dqkv, dp, _ = mha.bwd_nl(dout, qkv, S_dmask, ctx.cu_seqlens, ctx.p_dropout, ctx.max_s)
        else:
            dqkv, dp = mha.bwd(dout, qkv, S_dmask, ctx.cu_seqlens, ctx.p_dropout, ctx.max_s)

        return dqkv, None, None, None, None, None, None

class FMHA(torch.nn.Module):

    def __init__(self, config):

        super(FMHA, self).__init__()

        self.p_dropout = config.attention_probs_dropout_prob
        self.h = config.num_attention_heads
        self.hidden_size = config.hidden_size
        self.d = self.hidden_size // self.h
        assert self.d * self.h == self.hidden_size, "Invalid hidden size/num_heads"

    def forward(self, qkv, cu_seqlens, max_s, is_training=True):

        ctx = FMHAFun.apply(qkv.view(-1, 3, self.h, self.d), cu_seqlens, self.p_dropout, max_s, is_training)

        return ctx.view(-1, self.hidden_size)