[nnc] Only lower float conv2d's (#56289)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56289
While there's no reason to think non-float32 conv2d's *don't* work,
they're only tested in float32 now. Since that's the most important use case,
I'd rather restrict the dtypes than spend time testing all the weird dtype
combinations that could possibly happen.
ghstack-source-id: 126755549
Test Plan: unit tests
Reviewed By: navahgar
Differential Revision: D27828495
fbshipit-source-id: fcf179207f2c9b20e0e86eb2b85687517d87063c
