Added ATen exported targets for consuming projects

[willyd]

I added basic support for exported targets to make it easier for consuming projects to link to ATen. This is especially useful on Windows since both Release and Debug will work with the same install tree.

We may want to use target_link_libraries(ATen PRIVATE ...) with some dependencies but this is a start to make ATenConfig.cmake more complete.

[ezyang]

@pytorchbot test this please

[willyd]

I fixed the build error. This was caused by the THD_deps.txt file getting the name of the ATen target instead of the path to the library itself. Should be fixed now.

[ezyang]

@pytorchbot test this please

[ezyang]

@lantiga Is this PR still relevant?

[lantiga]

I think so, e.g. we can use this to load the aten cmake vars in the libtorch build instead of guessing.
Right now we just assume standard *nix install locations, which means that libtorch won’t build on Windows.

[willyd]

Sorry I did not notice that the CI builds were failing. I rebased on master once more and I believe I fixed all the issues the previous commits had.

[willyd]

@pytorchbot Test this please. Not sure if this will trigger the CI builds...

[ezyang]

@pytorchbot test this please

[willyd]

@ezyang I believe the CI builds should pass with the latest commit.

[ezyang]

@pytorchbot test this please

[ezyang]

@pytorchbot retest this please

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