Support lowering mhlo.gather to linalg

[GMNGeoffrey]

The only supported lowering of mhlo.gather to Linalg is via the subset that can be lowered to torch_index_select (which is then lowered to Linalg)

One approach would be to decompose this mondo-op with some transposes and reshapes to drop the extra attributes that basically just accomplish that. This should probably be a separate pass, since not all backends will necessarily be able to lower that into performant code, but it would certainly make lowering to linalg easier.

I may work on this if we decide we want it badly enough in IREE in the near future.

[GMNGeoffrey]

I'm looking into this