[TVMC][UMA] Support using UMA with TVMC

[rafzi]

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This change introduces:

• A TVMExtension interface to extend TVMC with custom code. Currently its only use is to add UMABackends.
• A command line option --experimental-tvm-extension <dir> to point to extension modules.
• An envvar $TVM_EXTENSION_DIR for the same purpose.

See discussion thread: https://discuss.tvm.apache.org/t/tvmc-uma-integration/14309

@Mousius @cgerum @MichaelJKlaiber @PhilippvK

[tvm-bot]

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• cc @Mousius, @gromero, @leandron, @lhutton1 _{See #10317 for details}

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[PhilippvK]

@tvm-bot rerun

[rafzi]

Sorry for the late update.

The outstanding test failure is likely because of the outstanding issue that the sys.path is modified in order to support all module loading. This probably inteferres with running the tests in parallel.

https://github.com/tum-ei-eda/tvm/blob/c1c2d3ca2db1c56d3cc8d695530ab7aa8b90ea00/python/tvm/driver/tvmc/extensions.py#L24-L34

I'm not sure how to resolve this as I did not find another method to achieve the same kind of module loading. Do we need to compromise here and settle for a simple module load? I unresolved the relevant discussion with @lhutton1.

@cgerum @PhilippvK