add python linking dependency only when USE_PYTHON=on

[nihui]

[facebook-github-bot]

:pill: CI failures summary and remediations

As of commit fb771ef5fe (more details on the Dr. CI page):

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• 3/3 failures introduced in this PR

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:detective: 2 new failures recognized by patterns

The following CI failures do not appear to be due to upstream breakages:

cmake_linux_cpu (1/2)

Step: "packaging/build_cmake.sh" (full log | diagnosis details | :repeat: rerun)

./hello-world: error while loading shared libra...open shared object file: No such file or directory

-- Configuring done
-- Generating done
-- Build files have been written to: /root/project/examples/cpp/hello_world/build
+ [[ linux-gnu == \m\s\y\s ]]
+ make -j8
Scanning dependencies of target hello-world
[ 50%] Building CXX object CMakeFiles/hello-world.dir/main.cpp.o
[100%] Linking CXX executable hello-world
[100%] Built target hello-world
+ ./hello-world
./hello-world: error while loading shared libraries: libpng16.so.16: cannot open shared object file: No such file or directory


Exited with code exit status 127

cmake_linux_gpu (2/2)

Step: "Build torchvision C++ distribution and test" (full log | diagnosis details | :repeat: rerun)

./hello-world: error while loading shared libra...open shared object file: No such file or directory

-- Configuring done

-- Generating done

-- Build files have been written to: /home/circleci/project/examples/cpp/hello_world/build

+ [[ linux-gnu == \m\s\y\s ]]

+ make -j8

[35m[1mScanning dependencies of target hello-world[0m

[ 50%] [32mBuilding CXX object CMakeFiles/hello-world.dir/main.cpp.o[0m

[100%] [32m[1mLinking CXX executable hello-world[0m

[100%] Built target hello-world

+ ./hello-world

./hello-world: error while loading shared libraries: libpng16.so.16: cannot open shared object file: No such file or directory



Exited with code exit status 127

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1 failure not recognized by patterns:

Job	Step	Action
cmake_macos_cpu	curl -o conda.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-MacOSX-x86_64.sh
sh conda.sh -b
source $HOME/miniconda3/bin/activate
conda install -yq conda-build cmake
packaging/build_cmake.sh
	:repeat: rerun

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[de-vri-es]

Without this, if the project is built without -DUSE_PYTHON=ON, anyone doing find_package(TorchVision) will get this error:

-- Configuring done
CMake Error at /usr/share/cmake/TorchVision/TorchVisionConfig.cmake:61 (set_target_properties):
  The link interface of target "TorchVision::TorchVision" contains:
    Python3::Python
  but the target was not found.  Possible reasons include:
    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

[YosuaMichael]

This seems to be related to https://github.com/pytorch/vision/issues/5863 cc @datumbox, Do you know who can review this PR?

[datumbox]

Right now our CI is still broken due to PyTorch upstream issues, so it's not a good time to test it. Let's wait until they are resolved (tracked at #5881) and then check what the CI things about them.

@mthrok Would you be OK to provide input on this as you have good knowledge of cmake?

[de-vri-es]

I was disappointed to see 0.13 released without this fix :(

Is there anything needed to get this moving?