Handle kwargs and not serializable values as configuration attributes #657

Context: #655.

A kwargs argument was added to schedulers in d7dcba4 just to display a deprecation warning. kwargs was interpreted as a configurable attribute, but it could not be serialized. This broke:

• Pipeline loading, via __repr__ in the warning message.
• Pipeline saving.

We just added a quick fix, but there were other ideas to address it. @anton-l proposed the following in

ignore = getattr(self, "ignore_for_config", [])
ignore.append("kwargs")

There is also another check in

Perhaps we should do this in a more general way and prevent accepting attributes that cannot be serialized, informing the user about the problem. A similar thing happened for dtype and is bound to happen again.

My first approach would be to attempt a JSON serialization in register_to_config and go from there, but I'm not fully familiar with the details of the code. Any thoughts?