Add variance/std when displaying evaluations with repetitions

[Bradley-Butcher]

Feature request

Currently only the mean of the metric is displayed in the UI when repetitions > 1. It would be nice to have variance/std.

Motivation

Many LLM APIs (including OpenAI) can struggle to give deterministic completions. It would be nice to see the stability of metrics without looking at the individual repetitions.

[hinthornw]

Ya have been wanting this myself. We'll put it on the roadmap - as of now I can't promise a timeline