[inductor] Do not reuse buffers across scopes in mem planning (#120777)
Summary: Previously, in the `memory_plan_reuse` we assumed that the generated code is flat: in the sense of it can't have nested scopes. However, with nested control flow codegen-ing, this is no longer the case. This causes bugs in buffers being reused across the visibility boundaries in different nested scopes.
In this PR, we add nested planning states in `memory_plan_reuse` on entering and exiting scope in the codegen. This restricts the buffer reusability only to the currently active (peak) scope / planning state.
Test Plan:
```
python test/inductor/test_control_flow.py -k test_subgraphs_with_parameters
...
----------------------------------------------------------------------
Ran 27 tests in 149.413s
OK
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120777
Approved by: https://github.com/chenyang78, https://github.com/desertfire, https://github.com/jansel
ghstack dependencies: #120665
