[NFC] Clean-up some old workarounds in sampler, simplify some code, and add error checking

Created by: PhilipVinc

This PR does the following minor changes:

• removes the jit function trampolines used by samplers to jit the code, as it's no longer needed thanks to improvements in jax caching mechanism, making the code easier to read
• remove some custom __repr__ definitions for samplers, in favour of using show_repr=False on n_chains of the samplers. This cuts some more LOCs.
• Makes MetropolisRule an abstract base class (it was raising errors anyway, and this makes it nicer in generated docs).

The following major changes:

• move nk.sampler.MetropolisRule abstract base class to nk.sampler.rules.MetropolisRule. In a later PR I'd like to deprecate the original binding. part of the reason is to make the metropolis.py file a bit smaller.
• Add some error checking code to MetropolisSampler, so that if one defines a custom rule or uses a model that returns the wrong shapes, it gives a more comprehensible error instead of imperscrutabili jax errors.