Refactor Pool.cpp (#2836)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2836

This change adds more lines than it subtracts, but it'll be worth it once we reuse the methods for `aten.convolution`.
ghstack-source-id: 221721759
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Reviewed By: copyrightly

Differential Revision: D55706057

fbshipit-source-id: 9113ed1a5374177263360dde89c626b8917c2378