Whilelist Transformers private method in DummyObject
# 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.
_The documentation is not available anymore as the PR was closed or merged._<|||||>You can't do that kind of check without triggerring recursion errors (cause it calls methods like `__getattribute__` inside that method ;-) ).