Use fun default composer run names

[irenedea]

Our previous default "llm" is depressingly literal, and because it does not change, it is hard to distinguish runs from the same environment.

https://github.com/mosaicml/composer/blob/main/composer/trainer/trainer.py#L549 composer generates some really fun and random run names, we should take advantage of this.

Testing

Tested on interactive by unsetting COMPOSER_RUN_NAME and RUN_NAME env vars.

created a run called 1729664919-daring-lori

[mvpatel2000]

https://github.com/mosaicml/llm-foundry/blob/c681a9c48002c5b7b1c847e264427ae6c9fbbd04/llmfoundry/command_utils/train.py#L325C68-L325C80

this variable can now be relaced with run_name is not None

env_run_name is also no longer needed and can be one-lined

[mvpatel2000]

I love fun