Fixed Polar GP example

[trsav]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've formatted the new code by running poetry run pre-commit run --all-files --show-diff-on-failure before committing.
• I've added tests for new code.
• I've added docstrings for the new code.

Description

Please describe your changes here. If this fixes a bug, please link to the issue, if possible.

Polar GP example was non-continuous through 0 - 2\pi. When defining the half-period constant c as follows:

    period: float = static_field(2 * jnp.pi)
    tau: float = param_field(jnp.array([4.0]), bijector=bij)

    def __post_init__(self):
        self.c = self.period / 2.0

c was still training as a parameter during optimisation and wasn't being considered static, resulting in an invalid Polar GP (I believe). Therefore I moved the definition of c to within the call method of the kernel, maintaining it as a constant during hyper-parameter optimisation. I'm not 100% sure on why this behaviour is happening, with training a constant, but this fixes it.

I have changed the docs to reflect this, also fixed a minor typo (circular_posterior), and changed linewidth of the fill_between to be 0 so there wasn't a funky line between 0 and 2/pi.

Issue Number: N/A

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