[doc] add a note about unit-testing to CONTRIBUTING.md

[Fridge003]

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As is discussed in the referred issue, if the version of python is too high (>=3.10) during establishing environment for unit testing, fbgemm-gpu==0.2.0 cannot be correctly downloaded.

So I'm adding a note in CONTRIBUTING.md to warn about this potential bug, so contributors can avoid wasting time on this requirement bug.

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