Add gpu::HostUnregisterOp

Without explicitly unregistering you will get

```
'cuMemHostRegister(ptr, sizeBytes, 0)' failed with 'CUDA_ERROR_HOST_MEMORY_ALREADY_REGISTERED'
```

in CUDA (for example) after repeated runs (e.g., during benchmarking the same kernel).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D147277