[profiler] Support top-level memory events (#51421)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51421
Mark memory events that did not happen within an operator context
explicitly in the profiler output.
Test Plan: python test/test_profiler.py -k test_memory_profiler
Reviewed By: ngimel
Differential Revision: D26166518
Pulled By: ilia-cher
fbshipit-source-id: 3c14d3ac25a7137733ea7cc65f0eb48693a98f5e
