[NNC] Bounds Inference: make inferred bounds respect gaps (#42185)
Summary:
A heavy refactor of bounds inference to fix some issues and bugs blocking using it to analyze cross thread interactions:
* We were merging all accesses to a Buf into a single bounds info entry, even if they did not overlap. E.g. if we accessed a[0:2] and a[5:6] we would merge that into a bound of a[0:6]. I've changed this behaviour to merge only overlapping bounds.
* We were not separating bounds of different kinds (e.g. Load vs Store) and would merge a Store bounds into a Load bounds, losing the information about what kind of access it was. E.g. this loop would produce bounds: [{Load, 0, 10}] and now produces bounds [{Load, 0, 9}, {Store, 1, 10}]:
```
for i in 1 to 10...
x[i] = x[i-1]
```
* Both ComputeAt and Rfactor relied on the overzealous merging and only used a single entry in the bounds list to determine the bounds of temporary buffers they created, which could result in temporary buffers allocated smaller than accesses to them. I've fixed Rfactor, but *not* ComputeAt - however all ComputeAt tests still pass (may require loop fusion to trigger this issue) - I will come back to it.
Being more precise about bounds is more complex, rather than taking the minimum of starts and maximum of stops we now need to determine if two bounds overlap or are adjacent. There are many edge cases and so I've added a bunch of test coverage of the merging method.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42185
Reviewed By: mruberry
Differential Revision: D22870391
Pulled By: nickgg
fbshipit-source-id: 3ee34fcbf0740a47259defeb44cba783b54d0baa
