Sampler-wise tune stats are not recognized & `InferenceData` left with 0 draws

[michaelosthege]

In pymc-devs/pymc#6835 (released in PyMC v5.7.0) the emission of the "tune" stat was changed to be a sampler-wise thing, which is why McBackend can no longer find it in the stored sample stats.

This triggered a second bug in .to_inferencedata(), where a np.repeat was used instead of a np.fill, leading to an empty array, that masked away all draws & stats.

Credits for the discovery go to @mamacneil via https://discourse.pymc.io/t/using-mcbackend-to-store-samples/13529.