code: def _pack(self, x):
      list_x = []
      for b in range(self.B):
          list_x_row = [
              x[b, t, : self.y_len[b] + 1] for t in range(self.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(self.f_len * (self.y_len + 1), dim=0)
      return x_packed, batch_offset
  
file_path: apex/apex/contrib/test/transducer/test_transducer_loss.py
imports: import unittest
  import torch
label_desc: Use image node_id 3 for calling the TransducerLossTest obj's underlying member method code with example usage: obj._pack(x) and returns: x_packed,  batch_offset