python: trim registration and loading of dialects and passes

[ashay]

In the interest of merging upstream LLVM quickly, a previous patch
(7f08169) updated the torch-mlir build to register all dialects and
passes through Python bindings. This patch limits the dialects and
passes to only those that are used in torch-mlir.

Key to this change are the removal of
MLIRPythonExtension.RegisterEverything and the introduction of a new
Python module (_mlir_libs/_site_initialize_0.py), where we register
the dialects and passes used by torch-mlir.

[silvasean]

Nice :)

[ashay]

I just now found out that the build error is due to the use of the default linker (ld?). I am using lld on my local machine, which doesn't produce the error. Still investigating.

[stellaraccident]

Thanks. I left some notes. I suspect that the C/C++ cross linking thing is actually what is tripping up ld, since what you are doing is introducing a kind of ODR violation.

I don't have time tonight, but if you struggle with this, let me know and I can straighten it out.

[stellaraccident]

Much better - thanks!

[ashay]

I suspect that the C/C++ cross linking thing is actually what is tripping up ld, since what you are doing is introducing a kind of ODR violation.

Thanks so much, that was spot on! I believe I have separated the C and C++ APIs now, but please let me know if you think more changes are necessary. Locally, using ld now works, so hopefully CI will pass as well.

[stellaraccident]

I suspect that the C/C++ cross linking thing is actually what is tripping up ld, since what you are doing is introducing a kind of ODR violation.

Thanks so much, that was spot on! I believe I have separated the C and C++ APIs now, but please let me know if you think more changes are necessary. Locally, using ld now works, so hopefully CI will pass as well.

I wish it didn't rely on such esoteric knowledge to spot such things. But glad it helped regardless :) Thanks for doing the adaptations! Much appreciated.

[powderluv]

@ashay could this be causing the nightly failure and also the

 ImportError: dlopen(/Users/kkiningh/Workspace/pytorch-mlir/torch-mlir/build/tools/torch-mlir/python_packages/torch_mlir/torch_mlir/_mlir_libs/_mlirRegisterEverything.cpython-39-darwin.so, 0x0002): symbol not found 
in flat namespace '_mlirRegisterAllDialects'  

reported on discord ?