Add test that returns number of registered vision ops

[bmanga]

Adds a simple executable that returns the number of registered torchvision ops. This should tell us whether the torchvision ops were successfully registered when <torchvision/vision.h> is included (pending PR #2798).

It's still missing integration in the CI as I am not familiar with that. The CI script should check that the program return value is > 0 (should return 9 for now).
@fmassa who should I ask for help in that regard?

@ezyang Do you think this suffices, provided we test enough plat/compiler combinations?

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[fmassa]

Thanks for the PR @bmanga !

For adding it together with CI, I think the easiest for now is to add a few more lines in

vision/packaging/build_cmake.sh

Lines 85 to 101 in 4db3dc6

	# Compile and run the CPP example
	popd
	cd examples/cpp/hello_world
	mkdir build
	cd build
	cmake .. -DTorch_DIR=$TORCH_PATH/share/cmake/Torch
	if [[ "$OSTYPE" == "msys" ]]; then
	"$script_dir/windows/internal/vc_env_helper.bat" "$script_dir/windows/internal/build_cpp_example.bat"
	cd Release
	else
	make
	fi
	# Run CPP example
	./hello-world

to compile and run the executable

If you need some more specific pointers on this, I think the right person to ask would be @andfoy

[bmanga]

@fmassa thanks for the pointer. How do I signal success/failure from that script? does exit 1 work as a failure indicator?

[fmassa]

@bmanga I would think so, although tests seems to have passed on Linux, so maybe there is something that we are missing here.

[bmanga]

@fmassa Indeed that's strange. I will have a look to see if the CI does something peculiar to make this work. Who is the person mainly responsible for the CI?

I can confirm that with my local setup (master torch, torchvision) the ops are not registered automatically, and I would assume I'm not the only one given the various torchvision issues.

[ezyang]

Probably better not to pass the count as the return code, it's just pretty weird and non-idiomatic. Print it and then test it from the scaffolding script. This may also fix your CI problems.

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