[fx2trt] Add support for torch.mean (#70052)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70052
As the title. Also refactored a bit to separate out the common part of adding a reduce operator.
This would make mnasnet lowerable without splitter.
Test Plan: Added unit tests.
Reviewed By: wushirong
Differential Revision: D33163950
fbshipit-source-id: 7eb8f8a852cd8e8d9937029c4b4602b036502b3a
