[ET-VK] Implement generic reduction shader + mean, sum, amax, amin (#6473)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6457

## Context

Introduce a generic shader to compute reduction along a single dim, and `keepdim = True`. With the generic shader template, `mean`, `sum`, `amin`, and `amax` can be implemented.
ghstack-source-id: 249709743
@exported-using-ghexport

Differential Revision: [D64840504](https://our.internmc.facebook.com/intern/diff/D64840504/)

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Co-authored-by: Stephen Jia <ssjia@meta.com>