[BUG] Early stopping does not seem to work

The early stopping callback does not work as expected, and crashes at the end of
max_epochs with:

```
~/miniconda3/envs/py38/lib/python3.8/site-packages/pytorch_lightning/trainer/callback_hook.py in on_train_end(self)
    155         """Called when the train ends."""
    156         for callback in self.callbacks:
--> 157             callback.on_train_end(self, self.lightning_module)
    158
    159     def on_pretrain_routine_start(self) -> None:

~/work/repos/prototorch_models/prototorch/models/callbacks.py in on_train_end(self, trainer, pl_module)
     18     def on_train_end(self, trainer, pl_module):
     19         # instead, do it at the end of training loop
---> 20         self._run_early_stopping_check(trainer, pl_module)
     21
     22

TypeError: _run_early_stopping_check() takes 2 positional arguments but 3 were given
```

prototorch/models/__init__.py

@@ -2,7 +2,8 @@
 
 from importlib.metadata import PackageNotFoundError, version
 
-from .callbacks import PruneLoserPrototypes
+from .callbacks import (EarlyStopWithoutVal, PrototypeConvergence,
+                        PruneLoserPrototypes)
 from .cbc import CBC, ImageCBC
 from .glvq import (
     GLVQ,