[ET-VK][EZ] Rename to max_pool2d.yaml

As ssjia 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`.

Differential Revision: [D55670408](https://our.internmc.facebook.com/intern/diff/D55670408/)

ghstack-source-id: 221058403
Pull Request resolved: #2820

Author: jorgep31415