[example] add llama2 example (#4527)

* [example] transfer llama-1 example

* [example] fit llama-2

* [example] refactor scripts folder

* [example] fit new gemini plugin

* [cli] fix multinode runner

* [example] fit gemini optim checkpoint

* [example] refactor scripts

* [example] update requirements

* [example] update requirements

* [example] rename llama to llama2

* [example] update readme and pretrain script

* [example] refactor scripts