Improve pack_sequence and pack_padded_sequence error message (#16084)
Summary:
Mention that if enforce_sorted=True, the user can set
enforce_sorted=False. This is a new flag that is probably hard to
discover unless one throughly reads the docs.
Fixes #15567
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16084
Differential Revision: D13701118
Pulled By: zou3519
fbshipit-source-id: c9aeb47ae9769d28b0051bcedb8f2f51a5a5c260
