DOC: pm.sample doesn't use ZarrTrace with numpyro, but docs don't say this

[brendan-m-murphy]

Issue with current documentation:

I tried out the ZarrTrace backend and didn't notice anything being written. A directory is created when I make the ZarrTrace object, but it isn't populated with any of the default group, and doesn't have the trace.

Looking through the code for pm.sample, I noticed that the backend isn't used if you specify e.g. nuts_sampler="numpyro". It would be nice to have this feature with numpyro, but perhaps for now it would be useful to mention in the docs that it only works with PyMC's samplers.

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