Handle `kwargs` and not serializable values as configuration attributes

[pcuenca]

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

diffusers/src/diffusers/configuration_utils.py

Line 408 in 534512b

ignore = getattr(self, "ignore_for_config", [])

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

There is also another check in

diffusers/src/diffusers/configuration_utils.py

Line 283 in 534512b

if "kwargs" in expected_keys:

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?

[patrickvonplaten]

I think the solution we have for now is good enough actually - what do you think @pcuenca ?

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[pcuenca]

Sounds good.