need fix BlockDiagonalMask error for xformer 0.0.28.post1

[SaiZyca]

PixArt and SANA need fix code for newer xformers

  if (xformers.__version__).split(".")[2] >= "28":
      attn_bias = xformers.ops.fmha.attn_bias.BlockDiagonalMask.from_seqlens([N] * B, mask)
  else:
      attn_bias = xformers.ops.fmha.BlockDiagonalMask.from_seqlens([N] * B, mask)