[pytorch] expose __ldg(const Half* ptr) to Clang in host mode (#38151)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38151
We need to expose this method to Clang unconditionally when building CUDA, otherwise it would error on device code calling `__ldg` with `Half*`.
Test Plan:
```
buck build -c fbcode.caffe2_use_mpi=1 -c fbcode.cuda_use_clang=true mode/opt //experimental/training_supercomputer/trainer/hpc_pt:trainer
```
Reviewed By: ngimel
Differential Revision: D21481297
fbshipit-source-id: aacfe7de2cdc8542908249081ddb58170b1e35ff
