Add Heteroscedastic likelihood

[thomaspinder]

Checklist

• I've formatted the new code by running uv run poe format before committing.
• I've added tests for new code.
• I've added docstrings for the new code.

Description

This PR introduces the ability to parameterise the noise term of the likelihood with a GP. Through a subsequent VI routine, a heteroscedastic model may then be inferred to capture a non-constant noise process.YX

Issue Number: N/A

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