Make package stable against CUDA updates

[enrico-lattuada]

Description CUDA Toolkit updates might break the links to DLLs in Windows. Make them more stable.

Proposed solution Add the CUDA Toolkit version to the _config.py file and read it to add the path in Windows.

Additional context See what pytorch does in the __init__.py:

...
cuda_version_1 = cuda_version.replace('.', '_')
cuda_path_var = 'CUDA_PATH_V' + cuda_version_1

and in the version.py.tpl (which is configured during install):

cuda = '{{CUDA_VERSION}}'

In our case, we want to use CUDAToolkit_VERSION, which is set by FindCUDAToolkit.

[enrico-lattuada]

Closed by #184