Fix torchvision install due to zippeg egg

[fmassa]

Fixes #1489

When compiling torchvision with zip_safe=True via python setup.py install, setuptools would generate a zippeg egg file. This made it impossible for custom extensions (which have recently been added) to work, because they would need to be unzipped beforehand.

This used to work with C++ extensions which relied on pybind11 because they were proper Python extensions, which were imported via import torchvision._C, and setuptools generated special code for handling it. This is not the case anymore with custom ops, which are just libs without any Python information.

Thanks for @tangorn and @pedrofreire for raising up the issue and giving a repro.

[vincentqb]

The tests that are failing don't seem related. LGTM.

[ezyang]

This is fine to merge, but I have a meta question which is why torchvision is using custom extensions instead of having setup.py manage compilation of all C code.

[fmassa]

@ezyang torchvision makes use of setup.py to manage the compilation of all C code.

The issue is that in some contexts, installing torchvision would yield a zippeg egg file.

Python knows how to handle this zippeg egg file for standard python extensions (when doing import torchvision._C for example), because it generates a python import code _C.py which handles the zip file.

But for torch ops (which rely on torch.ops.load_library(path)) this is not enough because we would need to unzip the egg file to get the full path for the _C.so lib that torch.ops.load_library expects.