feat: rename spread_squared to ensemble_spread, use std instead of var

[Kirwa-web]

Describe your changes

Renames the spread_squared metric to ensemble_spread and changes the
computation from variance (torch.var) to standard deviation (torch.std),
making the metric name and behaviour consistent with how ensemble spread is
conventionally reported.

Issue Link

Closes #62

Type of change

• ✨ New feature (non-breaking change that adds functionality)

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[Sir-Sloth-The-Lazy]

we have an open PR on this , It would be a duplicate work if you work on this seperately, kind of a waste for your efforts. Please check PR #226

[sadamov]

yeah the overlap here is clear, we also recently consolidated efforts on the prob. piping: #649 would be great to get your input directly in #226 @Kirwa-web

[Kirwa-web]

Good catch..from outside it's hard to see all the in-flight branches. I'll read through #226 and #649 before touching this area again, rather than add another parallel effort.