nested config update in _from_pretrained and _from_config

[ArneBinder]

With this PR, it is now possible to overwrite individual config entries that are arbitrarily nested. Does work for dicts, but not for lists.

This also adds the parameter config_override to _from_pretrained and _from_config to provide fine-grained control over this behavior. It accepts a bool value or a dict mapping from config keys to bool or dict values again, this can be recursive. If True, the value for the respective key (or the whole config, on the top level) is taken from the keyword arguments. If False, the keyword argument is discarded and the respective config value is taken. If it is None or if no bool value is set for the respective key, the keyword arguments are merged recursively into the config.

TODO:

• test in downstream setup