Expose Configuration Options for Perplexity calculations

[haileyschoelkopf]

At some point we should add options for PPL calculations, such as the ability to determine whether the windows overlap with each other, and .

Note that this heavily impacts the speed / compute required for evaluations in addition to scores.

Preferred implementation approach:

• convert the generation_kwargs TaskConfig field to request_kwargs, leaving back-compatibility for providing generation_kwargs if request_kwargs is not in a given config
• allow loglikelihood_rolling requests to take these request_kwargs, and implement new helper functions different from the current creation of context windows for loglikelihood rolling, and add unit tests for these helper fns
• add some example task using these options