[pytorch][aten][cuda] move CUDAGeneratorImpl.h to ATen/cuda (#70650)
Summary:
This patch moves a CUDA-specific file, `CUDAGeneratorImpl.h` to `ATen/cuda` as the following TODO comment in `CUDAGeneratorImpl.h` suggests:
```
// TODO: this file should be in ATen/cuda, not top level
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70650
Reviewed By: jianyuh, xw285cornell
Differential Revision: D33414890
Pulled By: shintaro-iwasaki
fbshipit-source-id: 4ff839205f4e4ea4c8767f164d583eb7072f1b8b
