torch.nn.utils.rnn.pack_padded_sequence
torch.nn.utils.rnn.pack_padded_sequence(input, lengths, batch_first=False, enforce_sorted=True)
Packs a Tensor containing padded sequences of variable length.
"input" can be of size "T x B x " where T is the length of the
   longest sequence (equal to "lengths[0]"), "B" is the batch size,
   and "" is any number of dimensions (including 0). If "batch_first"
   is "True", "B x T x *" "input" is expected.
For unsorted sequences, use enforce_sorted = False. If
   "enforce_sorted" is "True", the sequences should be sorted by
   length in a decreasing order, i.e. "input[:,0]" should be the
   longest sequence, and "input[:,B-1]" the shortest one.
   enforce_sorted = True is only necessary for ONNX export.
Note:
 This function accepts any input that has at least two dimensions.
 You can apply it to pack the labels, and use the output of the
