code: def backward(ctx, dout, *args):
      (
          q,
          k,
          v,
          out,
          softmax_lse,
          cu_seqlens,
          rng_state,
      ) = ctx.saved_tensors
      qkv_shape = q.shape[:-2] + (3, *q.shape[-2:])
      dqkv = torch.empty(qkv_shape, dtype=q.dtype, device=q.device)
      _flash_attn_varlen_backward(
          dout,
          q,
          k,
          v,
          out,
          softmax_lse,
          dqkv[:, 0],
          dqkv[:, 1],
          dqkv[:, 2],
          cu_seqlens,
          cu_seqlens,
          ctx.max_seqlen,
          ctx.max_seqlen,
          ctx.dropout_p,
          ctx.softmax_scale,
          ctx.causal,
          ctx.window_size,
          rng_state=rng_state,
      )
      dqkv = dqkv[
          ..., : dout.shape[-1]
      ]  # We could have padded the head dimension
      return dqkv, None, None, None, None, None, None, None
  
imports: from typing import Optional, Union
  import torch
  import torch.nn
  import flash_attn_2_cuda
label_desc: Use image node_id 2 for calling the FlashAttnVarlenQKVPackedFunc obj's underlying member method code with example usage: obj.backward(ctx, dout) and returns: dqkv,  None,  None,  None,  None,  None,  None,  None