feat: add MSE evaluation metric

[Hungreeee]

Description

This PR adds a new MSE metric for Pruna's evaluation framework, along with the relevant unit tests for it.

Related Issue

Fixes #388

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactor (no functional change)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Testing

• I added or updated tests covering my changes
• Existing tests pass locally (uv run pytest -m "cpu and not slow")

For full setup and testing instructions, see the Contributing Guide.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code, especially for agent-assisted changes
• I updated the documentation where necessary

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[sdiazlor]

Hi @Hungreeee. Thanks for the PR!