[NNC] Intermediate allocs flattened and dependency support (#49554)
Summary:
Makes two changes in NNC for intermediate buffer allocations:
1. Flattens dimensions of buffers allocated in LoopNest::prepareForCodegen() to match their flattened usages.
2. Adds support for tracking memory dependencies of Alloc/Free to the MemDependencyChecker, which will allow us to check safety of accesses to intermediate buffers (coming in a future diff).
I didn't add any new tests as the mem dependency checker tests already cover it pretty well, particularly the GEMM test.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49554
Reviewed By: VitalyFedyunin
Differential Revision: D25643133
Pulled By: nickgg
fbshipit-source-id: 66be3054eb36f0a4279d0c36562e63aa2dae371c
