Change codecov comment style to be less verbose (#46499)
Summary:
Do not post comments if there are no changes in coverage and post only diff stats
Partially addresses an issue raised in https://github.com/pytorch/pytorch/issues/44187
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46499
Reviewed By: walterddr
Differential Revision: D24373480
Pulled By: malfet
fbshipit-source-id: a49d7e661507ad98d5222c119b2f3f3550c1a949
