[mlir][linalg] Implement patterns for reducing rank of named linalg contraction ops

[srcarroll]

This patch introduces pattern rewrites for reducing the rank of named linalg contraction ops with unit spatial dim(s) to other named contraction ops. For example linalg.batch_matmul with batch size 1 -> linalg.matmul and linalg.matmul with unit LHS spatial dim -> linalg.vecmat, etc. These patterns don't support reducing the rank along reduction dimension as those don't convert to other named contraction ops.

[ftynse]

A bunch of nits, LGTM otherwise.

[j2kun]

This is a neat feature! Any objection into making it a standalone pass?

[srcarroll]

This is a neat feature! Any objection into making it a standalone pass?

Thanks. No objection at all. If I remember correctly the pattern rewrites are public so they should be able to be reused by any pass. In fact someone requested a feature to have options to choose what ops are targeted for reduction (or choose how low to take the rank). I was going to but haven't had time so if you want to address that too that would be great.

[srcarroll]

the pattern rewrites are public

correction. only the populate patterns function is public. so we can either make the pattern rewrite public to make it easier to build passes with individual patterns, or just add the options to the populate function. i dont think i have a particular preference