Investigate if it's possible to build only CPU version without CUDA

[pbelevich]

If CUDA is not available it make sense to have an option to build CPU-only version

[pbelevich]

depends on pytorch/pytorch#39618

[ilyamironov]

+1

When I launched setup.py on my Mac (no CUDA), it failed with

Traceback (most recent call last):
  File "setup.py", line 20, in <module>
    'nvcc': ['-O2', '--expt-extended-lambda', '-DAT_PARALLEL_OPENMP=1']})],
  File "/Users/imironov/Library/Python/3.7/lib/python/site-packages/torch/utils/cpp_extension.py", line 705, in CUDAExtension
    library_dirs += library_paths(cuda=True)
  File "/Users/imironov/Library/Python/3.7/lib/python/site-packages/torch/utils/cpp_extension.py", line 795, in library_paths
    if (not os.path.exists(_join_cuda_home(lib_dir)) and
  File "/Users/imironov/Library/Python/3.7/lib/python/site-packages/torch/utils/cpp_extension.py", line 1652, in _join_cuda_home
    raise EnvironmentError('CUDA_HOME environment variable is not set. '
OSError: CUDA_HOME environment variable is not set. Please set it to your CUDA install root.

[pbelevich]

@ilyamironov implemented in #28
python setup install now automatically detects if CUDA is available, if not - builds just CPU version