Fix from_pretrained with default base_model_prefix
# What does this PR do?

There is a subtle bug in `from_pretrained` that was discovered by @NielsRogge with the Enformer model. Basically, `from_pretrained` will silently not load anything when a subclass does not define `base_model_prefix`. This is because the defaults is left at `""` which then:
- sets `has_prefix_module` to `True`
- changes the `prefix` to `"."` since the model does not have any attribute named `""`
- then ignore all weights

This PR fixes that and removes the `base_model_prefix` attribute from the custom modeling classes used during the test, to make sure we don't have any regression on this.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ah yeah that makes sense! Thanks a lot!