ifdef guard some explicit pragma unrolls (#19018)
Summary:
the ROCm compiler cannot and will not satisfy them, causing compile time warnings.
Reason being a runtime loop trip count.
Some warnings remain arising from other parts of the ROCm stack - tickets are filed and they will be resolved within these components.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19018
Differential Revision: D14832859
Pulled By: ezyang
fbshipit-source-id: 0d66e4aebe4e56af14dd5e2967d3c374a82be25c
