Can't store object dtyped sampler warnings in ClickHouseBackend

[michaelosthege]

Since PyMC v4.3.0 the sampler warnings are collected as stats, which means that they end up in the sampler_stats that the ClickHouseBackend is asked to collect.

This leads to problems because object typed variables can't be stored in ClickHouse.

How this could be addressed:

• Turn off collection of warning stats at the PyMC level
• Filter them in the TraceBackend
• Have ClickHouseBackendignore them

Not having strong preferences yet, but the best way might be to have the ClickHouseBackend.init_run filter the unsupported variables out.