[Profiler] Memory profiler part 9: Mark activations (#88924)
This is a fairly straightforward pass: start at inputs and flood fill until we reach the backward pass.
Differential Revision: [D40868662](https://our.internmc.facebook.com/intern/diff/D40868662/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88924
Approved by: https://github.com/chaekit
