code: def _pack(self, x, f_len, g_len):
      B = x.size(0)
      list_x = []
      for b in range(B):
          list_x_row = [x[b, t, : g_len[b]] for t in range(f_len[b])]
          x_row = torch.cat(list_x_row)
          list_x.append(x_row)
      x_packed = torch.cat(list_x).data.clone()
      x_packed.requires_grad = True
      batch_offset = torch.cumsum(f_len * g_len, dim=0)
      return x_packed
  
imports: import unittest
  import torch
label_desc: Use image node_id 3 for calling the TransducerJointTest obj's underlying member method code with example usage: obj._pack(x, f_len, g_len) and returns: x_packed