Feat (brevitas_examples/llm): support for lighteval

[Giuseppe5]

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[nickfraser]

We plan to make quantize_llm return a dictionary with all results (Float/Quant PPL as well as any few_shot tasks). Does lighteval have its result accessible in a dictionary somehow? If this is available somewhere, it would make the future PR much easier.

[nickfraser]

Only one comment about an output dictionary, otherwise LGTM!