Add new events

## 🚀 Feature

Idea is to add supplementary events to Checkpoint/ModelCheckpoint, EarlyStopping  and maybe some others:
```python
class Checkpoint:

    def __call__(self, engine):
        # ...
        self.save_handler(checkpoint, filename)
        # TRIGGER new EVENT
        engine.fire_event(Checkpoint.SAVED_CHECKPOINT)
```


