[ET-VK][ez] Apply rotary embedding as Module (#6422)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6391

## Context

As title. Wrap the `apply_rotary_emb` function call in a `nn.Module` to make it easy to perform a source module replacement for rotary embedding calculation.

The Vulkan delegate will use the source module replacement technique to insert a custom op to calculate rotary embeddings.
ghstack-source-id: 249175724
@exported-using-ghexport

Differential Revision: [D64697589](https://our.internmc.facebook.com/intern/diff/D64697589/)

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