Add support for flexible Python3 versions

[raghavmecheri]

The installation process seems to currently depend on python3.8 being installed on the user's system. I'd propose one of two alternatives:

1. Relax this dependency to just a generic Python3 dependency. This could be accomplished by changing the Python version specified in publish-prebuild.yml to just 3.x
2. Use a build matrix and build for multiple Python versions (3.5, 3.6, 3.7, 3.8, 3.9)

@arition What do you think?

[arition]

For the first solution, I don't think it will solve the problem since the compiler will still try to link the binary to a specific python version that exists in the building environments.

For the second solution, for now I think the prebuild package has no ability to detect the Python version on target system, so it is not helpful to build the binary for multiple python versions.

[arition]

Since the linking to python is completely useless in this project, I would like to create a patch for torchvision that disable ununsed python related code on compiling. With the upcoming Torchvision 0.8.x, we also need to do similar things to disable useless libjpeg / libpng linking.

[raghavmecheri]

Since the linking to python is completely useless in this project, I would like to create a patch for torchvision that disable ununsed python related code on compiling. With the upcoming Torchvision 0.8.x, we also need to do similar things to disable useless libjpeg / libpng linking.

This could work -- I'm assuming we do need at least a part of torchvision though, right?

[raghavmecheri]

Also, is the static dependency on Python 3.8 a new introduction? I didn't see it earlier

[arition]

Also, is the static dependency on Python 3.8 a new introduction? I didn't see it earlier

I think cmake will automatically find and link a python version installed on your system, so you may not noticed before.

[arition]

Just make some small experiments. Seems that not only Torchvision, PyTorch itself also has a reference on Python. Again, I do not have a Mac, so I cannot do further experiments. If you have time, can you use otool to check if pytorch's dylib has a reference/link to python library?

[raghavmecheri]

Just make some small experiments. Seems that not only Torchvision, PyTorch itself also has a reference on Python. Again, I do not have a Mac, so I cannot do further experiments. If you have time, can you use otool to check if pytorch's dylib has a reference/link to python library?

Sorry for the delayed response on this, but it doesn't. Only libtorchvision.dylib does