Add launch bounds needed for ROCm 2.0 (#15400)
Summary:
ROCm 2.0's compiler requires launch_bounds annotations if flat work group sizes are larger than the default of 256.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15400
Differential Revision: D13531239
Pulled By: ezyang
fbshipit-source-id: c0b40600a8c332823da6c7113c644d8dba424a9c
