Added base_class.__init__ call to classes in mlp.py

Quite a few classes in mlp.py derive from Layer but never call the __init__ method on their base class. This means that Layer's base class, Model, is never initialized, which means self.names_to_del is never initialized to the empty set as it should be:

https://github.com/lisa-lab/pylearn2/blob/master/pylearn2/models/model.py#L394

```
        self.names_to_del = set()
        self._test_batch_size = 2
```

This was likely never the intended functionality, and it causes an error when one attempts to use the Model.register_names_to_del method the first time.

This PR adds a call to the base class __init__ method for all classes in mlp.py.