pack_padded_sequence: Check for empty (zero-element) tensors (#21461)
Summary:
Fixes: #20529
Thank you, JamieCT for the bug report with reproducing script.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21461
Differential Revision: D15696183
Pulled By: ezyang
fbshipit-source-id: a93cde2c924f8447563c64ce8a1cf75fcee60a01
