Preserve CUDA gencode flags (#41173)
Summary:
Add `torch._C._cuda_getArchFlags()` that returns list of architecture `torch_cuda` were compiled with
Add `torch.cuda.get_arch_list()` and `torch.cuda.get_gencode_flags()` methods that returns architecture list and gencode flags PyTorch were compiled with
Print warning if some of GPUs is not compatible with any of the CUBINs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41173
Differential Revision: D22459998
Pulled By: malfet
fbshipit-source-id: 65d40ae29e54a0ba0f3f2da11b821fdb4d452d95
