Revise memory monitor to align with Xcode metrics (#3131)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3131

Ref: https://forums.developer.apple.com/forums/thread/105088

*If you’re going to record a single number, this footprint value is a good one to use. I don’t think we guarantee that it’ll align with the Xcode memory gauge, but it’s much more useful value than all the older stuff (like resident_size)*

Therefore revise it.

Reviewed By: shoumikhin

Differential Revision: D56290391

fbshipit-source-id: f3f408f2677c3788a25e46af0df0eccd23a21c2f