Add RPATH to torchvision cmake target for torch libraries

[bmanga]

This adds the RPATH to the torch libraries so that they can be found when a project links to torchvision

[vfdev-5]

@bmanga I checked this PR with the test from #2807 on MacOSX. The example works on the current master and on this PR giving 9 ops. The only difference is with linked libraries:

# master

libtorchvision.dylib:
        @rpath/libtorchvision.dylib (compatibility version 0.0.0, current version 0.0.0)
        @rpath/libc10.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/local/opt/libpng/lib/libpng16.16.dylib (compatibility version 54.0.0, current version 54.0.0)
        /usr/local/opt/jpeg/lib/libjpeg.9.dylib (compatibility version 13.0.0, current version 13.0.0)
        /usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/Python (compatibility version 3.7.0, current version 3.7.0)
        @rpath/libtorch.dylib (compatibility version 0.0.0, current version 0.0.0)
        @rpath/libtorch_cpu.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.0.0)

and

# PR 2801

libtorchvision.dylib:
        @rpath/libtorchvision.dylib (compatibility version 0.0.0, current version 0.0.0)
        @rpath/libc10.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/local/opt/libpng/lib/libpng16.16.dylib (compatibility version 54.0.0, current version 54.0.0)
        /usr/local/opt/jpeg/lib/libjpeg.9.dylib (compatibility version 13.0.0, current version 13.0.0)
        @rpath/libpython3.7m.dylib (compatibility version 3.7.0, current version 3.7.0)
        @rpath/libtorch.dylib (compatibility version 0.0.0, current version 0.0.0)
        @rpath/libtorch_cpu.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.0.0)

[bmanga]

@vfdev-5 Thanks for checking. Given that it seems to work fine also on osx, do you think it's fine to merge? I am not sure what is going on with the tests, but it doesn't seem to work for me locally without this change (I get a runtime error saying that the libtorch libraries could not be found).

[vfdev-5]

@bmanga I updated the PR to current master, so let's see what CI shows (we can skip Travis as it is broken now) and see if possible to merge like that.

[vfdev-5]

Good to merge. Travis failure is unrelated.
Thanks @bmanga !

[codecov]

Codecov Report

Merging #2801 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2801   +/-   ##
=======================================
  Coverage   73.36%   73.36%           
=======================================
  Files          99       99           
  Lines        8787     8787           
  Branches     1387     1387           
=======================================
  Hits         6447     6447           
  Misses       1916     1916           
  Partials      424      424           

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[fmassa]

Thanks @bmanga and @vfdev-5 !