fix: jax backend tolist for tracers in logging

[pfackeldey]

Description

This PR closes #1422

This failure is encountered during tracing time. JAX can not convert a tracer during .tolist (because it has no data to put in a list) which is why it fails with a different error. This PR checks if we're currently in tracing time and instead returns the tracer (or rather its abstract value for a little nicer representation).

The new error looks as expected like this now:

>>> import pyhf
>>> pyhf.set_backend("jax")
>>> pyhf.simplemodels.uncorrelated_background([10], [15], [5])
>>> pyhf.infer.mle.fit([12.5], m)
>>> ... InvalidPdfData: "eval failed as data has len 1 but 2 was expected"

The logging will print:

"Eval failed for data ShapedArray(float64[1]) pars: ShapedArray(float64[2])"

This is the most information available during tracing time (shape and dtype), there's not much more to give to the user.

Checklist Before Requesting Reviewer

• Tests are passing
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[kratsg]

this likely needs #2566 in