Rename to max_pool2d.yaml (#2820)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2820

As SS-JIA mentioned in the next change, we are following a convention where glsl and yaml files have a 1:1 mapping. To make the mapping clear, files should use the same name so `pool.yaml` should actually be `max_pool2d.yaml` to match `max_pool2d.glsl`.
ghstack-source-id: 221721756
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Reviewed By: copyrightly

Differential Revision: D55670408

fbshipit-source-id: 775f45f3b44a3bb58e653021ba6e095ee8aa9a3f