[Feature] Add a customizable timeout for GuideLLM

[thameem-abbas]

For ultra-long input sequence lengths, GuideLLM can take long times to prepare the samples for the benchmark. The default multiplier of 1.5x of test duration is insufficient in this case and we will need to provide the ability to tweak the timeout duration here.

[aas008]

@thameem-abbas Does this mean users should be able to set something like timeout_multiplier: 3.0 in their config?

[thameem-abbas]

How this timeout is set is definitely up for discussion. Or we could infer it in runtime from the baseline trial.