feat: err when unsupported attn impl is set w/ `--continuous_batching` (#40618)
* feat: err when unsupported attn impl is set w/ `--continuous_batching`
* refactor: move defaults and support list to CB code
* feat: add action item in error msg
* fix(serve): add default attn implementation
* feat(serve): add log when `attn_implementation` is `None`
* feat: raise Exception when attn_implementation is not supported by CB