code: def _unskew(X):
      """reverse _skew operation"""
      # X = B x M x L+M
      B, M, L = X.size()
      L -= M
      X = X.view(B, -1)  # B x ML+MM
      X = F.pad(X, (0, M))  # B x ML+MM+M
      X = X.view(B, M, M + L + 1)  # B x M x L+M+1
      X = X[:, :, :L]  # B x M x L
      return X
  
imports: import math
  import torch
  import torch.nn
  import torch.nn.functional
  from fairseq.modules.layer_norm import LayerNorm
  from .adaptive_span_attention import AdaptiveSpan
label_desc: Use image node_id 2 for calling a global function with example usage: _unskew(X) and returns: X