Add warmup for Llama (#5756)

Summary:
Load the model. Run the everything twice. Reset stats in between two runs. Also decrease logging level for warm up.

Notes:

Tested on Android and Mac. With Llama2 and Llama3 - with temperature=0 produces same output.
This warm up option is disabled by default.
This is inspired from llama.cpp options [[1](https://github.com/ggerganov/llama.cpp/blob/ea9c32be71b91b42ecc538bd902e93cbb5fb36cb/common/common.cpp#L897-L929), [2](https://github.com/ggerganov/llama.cpp/blob/ea9c32be71b91b42ecc538bd902e93cbb5fb36cb/examples/llama-bench/llama-bench.cpp#L1595-L1602)].
Sample [runs](https://www.internalfb.com/phabricator/paste/view/P1613261035)

Pull Request resolved: https://github.com/pytorch/executorch/pull/5756

Reviewed By: mcr229, metascroy

Differential Revision: D63642723

Pulled By: digantdesai

fbshipit-source-id: 39ff257eda182fff423f90582a9f32387cfdb253