Install tritonbench as a library

[xuzhao9]

Now users can import Tritonbench as a library:

$ pip install -e .
$ python -c "import tritonbench; op = tritonbench.load_opbench_by_name('addmm');"

Clean up the init file so that dependencies like os and importlib will not pollute the namespace.

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[FindHao]

LGTM!

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@xuzhao9 merged this pull request in bec6d9f.