Fix LTC lib_torch_mlir_ltc.so import error

[henrytwo]

Addresses #1271, which will allow LTC to work from the prebuild wheels without import issues.

Previously, we were missing some cmake macros to include $ORIGIN to the list of directories searched for shared libraries.

As part of this, the reference LTC backend + base LTC backend were moved to _mlir_libs to be consistent with our other libraries (and also because the both of the libs need to be in the same directory for linking).

cc: @ke1337 @antoniojkim

[henrytwo]

@silvasean In the future should we add some test that uses the built wheel to ensure we don't hit a similar issue in the future?

It looks like this issue stemmed from the fact that when the lib was used, it wasn't in the same location where it was originally built, which is it why it failed to find the shared library. Piping the binary thru strings, I could see some hardcoded paths.

With this PR, $ORIGIN was added to the list of paths searched for shared libs, which should solve the problem.

[silvasean]

Great, thanks!

[silvasean]

@silvasean In the future should we add some test that uses the built wheel to ensure we don't hit a similar issue in the future?

I think we attempt to do that here:

torch-mlir/.github/workflows/buildRelease.yml

Line 25 in a1ace06

- name: Build Python wheels and smoke test.

Maybe we can extend the "smoke test" that is done there to do a basic LTC test?