[Pytorch Profiler] Move start timestamp to end of start callback (#62191)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62191
This moves start timestamping to end of callback. This way we dont
account for callstack/module hierarchy related overhead in op runtime.
Test Plan:
CI
Imported from OSS
Reviewed By: ilia-cher
Differential Revision: D29910519
fbshipit-source-id: f462031a81ae12b3db7993cf482e5ad93a35e096
