Add options to conditionally include Python

[dfalbel]

Hi everyone!

Thanks for this amazing package!

I'm working on a port of pytorch_sparse for the R language so users can benefit of the sparse operators together with torch. The current version lives in this repo.
It would be nice though if one could use the C++ library without linking to Python.

This PR adds a CMake option WITH_PYTHON that when set to OFF allows the package to be built entirely without linking to Python. By default, we tried to keep the same behavior, link to Python and include Python.h headers.

[dfalbel]

I think the error to install pytorch is unrelated to this change. But we still might need to modify:

pytorch_sparse/setup.py

Line 37 in fe8c3ce

define_macros = []

to add the WITH_PYTHON definition.

[rusty1s]

Thank you. This looks good. Left a few comments.

As you pointed out, we also need to set WITH_PYTHON by default in setup.py.

[rusty1s]

Why do we now include torch.h rather than extension.h?

[dfalbel]

extension.h includes Python.h so at least in the non-python builds we can't have it.

https://github.com/pytorch/pytorch/blob/e88c999da38f30da123e14bdf0898bc475c8d04a/torch/extension.h#L1-L6

However the library doesn't seem to use any <torch/python.h> features as operators seem to be exported via RegisterOperators.

We could still keep including extensions.h in the Python builds and only remove them in the non-python builds if you think it's safer. What do you think?

[rusty1s]

Why do we need those?

[dfalbel]

For some reason, including torch/python.h leaks the getpid() headers. Those headers are not included in torch/torch.h so we had to manually add them as getpid() is used for random seed initialization in a few operators.

Since getpid() is OS specific we have to include different headers on Unix and Windows.

[codecov-commenter]

Codecov Report

Merging #196 (c6c2eb3) into master (fe8c3ce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #196   +/-   ##
=======================================
  Coverage   72.32%   72.32%           
=======================================
  Files          28       28           
  Lines        1120     1120           
=======================================
  Hits          810      810           
  Misses        310      310           

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[rusty1s]

Thank you!