code: def subsequent_mask(length):
      mask = (
          torch.tril(torch.ones(length, length))
          .unsqueeze(0)
          .unsqueeze(0)
      )
      return mask
  
imports: import math
  import numpy
  import torch
  from torch import nn
  from torch.nn import functional
label_desc: Use image node_id 14 for calling a global function with example usage: subsequent_mask(length) and returns: mask