phi2 conversion/optimization script (#19338)
### Description
<!-- Describe your changes. -->
This PR adds
onnx conversion script for dynamo exported phi2,
optimization script,
and inference example script
A readme file is added as documentation.
https://github.com/microsoft/onnxruntime/tree/wangye/phi2_doc/onnxruntime/python/tools/transformers/models/phi2#readme
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
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Co-authored-by: Edward Chen <18449977+edgchen1@users.noreply.github.com>
