feature(normalize gen_kwargs; add truncation_side to vllm)!

[baberabb]

closes #3505

• Added a utility to normalize gen_kwargs. Normalizes do_sample and temperature to be consistent across models.
• Added truncation_side=right|middle|left arg to vllm to use other than left truncation. Will see about adding this to huggingface as well, but slightly non-trivial as HF requires sampling params per batch while vllm can run different per sample.

closes #3505 where there was an inconsistency in sampling params between HF and vllm when both do_sample: false AND a non-zero temperature is specified in a task config. Now normalized to:

Config	Result
Nothing specified	Greedy (temp=0.0)
temperature: 0.8 (no do_sample)	Sampling (temp=0.8)
do_sample: false	Greedy (temp=0.0)
do_sample: false, temperature: 0.8	Greedy (temp forced to 0.0)
do_sample: true, temperature: 0.8	Sampling (temp=0.8)