pad_packed_sequence: doc improvement (#33768)
Summary:
pad_packed_sequence:
1. clarify that batch's order is restored to the original one
2. add example
This is a follow up to https://github.com/pytorch/pytorch/issues/33746
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33768
Differential Revision: D20102792
Pulled By: ngimel
fbshipit-source-id: 5ef511e5e3833edcb85cc01af0e92568b6d7a3cf
