Sparse CUDA: rename files *.cu -> *.cpp (#63894)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63894
This PR introduces a few code structure changes. There is no need to use
.cu extension for pure c++ code without cuda. Moved
`s_addmm_out_csr_sparse_dense_cuda_worker` to a separate cpp file from
cu file.
cc nikitaved pearu cpuhrsch IvanYashchuk ngimel
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D30548771
Pulled By: cpuhrsch
fbshipit-source-id: 6f12d36e7e506d2fdbd57ef33eb73192177cd904
