Move the description comment of compute_bucket_assignment_by_size from cpp to the header file. (#44703)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44703
The description of this public function should be in the header file.
Also fix some typos.
Test Plan: N/A.
Reviewed By: pritamdamania87
Differential Revision: D23703661
fbshipit-source-id: 24ae63de9498e321b31dfb2efadb44183c6370df
