Quiet continuous batching at default verbosity (#48314)
* Quiet continuous batching at default verbosity
A plain generate_batch printed five WARNING lines (attention auto-switch, warmup start/finish,
progress-bar note) and a progress bar; generate prints nothing. Status messages move to info,
and the progress bar is opt-in. Actionable warnings (unset eos_token_id, warmup failure, dropped
requests) stay at warning.
* Apply suggestion from @qgallouedec
* Apply review: keep the attn switch a warning, TODO for the warmup progress bar
