Attribution API refactor: Introduce an optional agg/element-wise variance to LLM attribution results

[craymichael]

Summary: As title. It is possible for attr to be computed as an estimated amount over multiple samples of the response, so the estimate has variance. This adds an attribute to store this variance in the results, if we have it.

Differential Revision: D84970183

[meta-codesync[bot]]

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