Allow Automodel to use from_config with custom code.

[DN6]

What does this PR do?

AutoModel currently doesn't support creating objects using from_config, but this can be useful in cases where we might want to load a custom component that doesn't have any weights associated with it e.g. Custom scheduler like components for pipelines.

Fixes # (issue)

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[yiyixuxu]

ohh I think maybe we should support this in from_pretrained()? even for scheduler and such weightless things, we load them using from_pretrained(), no?

we normally use from_config like this, would be confusing I think

pipe = DiffusionPipeline.from_pretrained(..)
pipe.scheduler = NEWSCHEDULER.from_config(pipe.config)