vllm sleep mode support

[ved1beta]

What does this PR do?

Adds

enable_sleep_mode=script_args.enable_sleep_mode,
sleep_timeout=script_args.sleep_timeout,

Fixes axolotl/135

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

[qgallouedec]

answered here: axolotl-ai-cloud/axolotl#3135 (comment)