Fix Funnel Transformer conversion script
# What does this PR do?

The conversion script was using the wrong kind of model, so wasn't working. I've also added the option to convert the base models.

Fixes #9644

Do not miss ```transformers/cammand/convert.py```  for ```transformer-cli``` user.
Need ```base_model``` arg for ```convert_tf_checkpoint_to_pytorch(self._tf_checkpoint, self._config, self._pytorch_dump_output)```.