[Memory Snapshot] Stop clearing history when changing context (#120436)
Summary:
This change will avoid clearing the memory event history, when changing the context from `record_memory_history(context=None)` to `record_memory_history(context="python")`.
Now it will continue recording memory events with changing context on the fly. Only `record_memory_history(enabled=None)` will clear the history.
Test Plan:
# Ran on the following local Resnet50 example:
- At iteration=0, record_memory_history(context=None, stacks="python")
- At iteration=3, record_memory_history(context="all", stacks="python")
- After iteration=4, export_memory_snapshot()
## Before:
- Only collects the last 2 iterations with python call stacks.
## After:
- Collects all 5 iterations, where first 3 iterations have no call stacks, and last 2 iterations have python call stacks.
Differential Revision: D54084017
Pulled By: aaronenyeshi
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120436
Approved by: https://github.com/zdevito, https://github.com/leitian
