Switch to per-shader YAML files (#2459)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2459

## Context

Instead of having an `all_shaders.yaml` file that defines variants for all shaders, switch to having a YAML file for each shader template. This serves two purposes:

1. Makes it easy to find the variants that will be generated for each shader template
2. Reduces the number of potential conflicts of multiple developers adding individual operators.

Reviewed By: copyrightly, jorgep31415

Differential Revision: D54960165

fbshipit-source-id: 0f92a61c404e5be1e2a1304d089bfbadf9f6c518