add phi-3-mini eager mode example (#4315)

Summary:
This PR adds an example script in eager mode which does inference with/without kv cache enabled. This comes handy when we want to verify that phi-3-mini works in eager mode.

Pull Request resolved: https://github.com/pytorch/executorch/pull/4315

Test Plan:
```
python3 -m examples.models.phi-3-mini.eager -s 128 -kv -p "Tell me a story"
python3 -m examples.models.phi-3-mini.eager -s 128 -p "Tell me a story"
```
Verify that the model runs faster with kv cache enabled.

Reviewed By: JacobSzwejbka

Differential Revision: D60061822

Pulled By: helunwencser

fbshipit-source-id: 483d2f9e56f9397f78dec805a0c1a110cb1cfc28