[THC] Rename THCTensorMathMagma from cu to cpp (#58521)
Summary:
This supposed to be a no-op (as .cu file do not contain any cuda code),
that reduces compilation time 2.5x:
```
$ time /usr/local/cuda/bin/nvcc /home/nshulga/git/pytorch/aten/src/THC/THCTensorMathMagma.cu -c ...
real 0m7.701s
$ time /usr/local/cuda/bin/nvcc /home/nshulga/git/pytorch/aten/src/THC/THCTensorMathMagma.cpp -c ...
real 0m2.657s
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58521
Reviewed By: ngimel
Differential Revision: D28526946
Pulled By: malfet
fbshipit-source-id: ed42a9db3349654b75dcf63605bb4256154f01ff
