UX Improvements (#2291)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2291

* Chat UX
* RAM info
* Local model selection
* Model load time
* Tokens/sec info

bypass-github-pytorch-ci-checks
CI broken

Reviewed By: shoumikhin

Differential Revision: D54621204

fbshipit-source-id: a12282d5dfd34770f5f6de46db6a6132eaf7d950