Whilelist Transformers private method in DummyObject

[sgugger]

What does this PR do?

Fixes #20671

As reported in #20671, calling AutoModel.from_config on a model with a missing specific soft dependency does not raise the appropriate error. This is because this method ends up calling ModelClass._from_config and the DummyObject class does not raise the error on all private attributes (basically because we need the __xxx__ attribute to stay the same). This PR whitelists _from_config to fix the issue.

[ydshieh]

Thank you!

(do you think it's even fine for us to call super().__getattribute__(key) whenever super has key as its attribute? i.e. not to check _ or a whitelist, but just check if super has that attr)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

You can't do that kind of check without triggerring recursion errors (cause it calls methods like __getattribute__ inside that method ;-) ).