Callback derived class is called without module argument
🐛 Bug
The class Callback(abc.ABC) API expects trainer and sometimes pl_module to be supplied. E.g.:
def on_init_start(self, trainer):
    pass

See the definition
However, the caller TrainerCallbackHookMixin calls the callback methods without the module argument. E.g.:
for callback in self.callbacks:
     callback.on_init_start(self)  # self is Trainer / TrainerCallbackHookMixin

See the calls
To Reproduce
Follow the documentation for callbacks
and expect similar output
Traceback (most recent call last):
  File "./train_encdec_pl.py", line 33, in <module>
    callbacks=[SavePredictionCB])
  File "/workspace/oplatek/code/demo/venv/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 239, in __init__
    self.on_init_start()
  File "/workspace/oplatek/code/demo/venv/lib/python3.6/site-packages/pytorch_lightning/trainer/callback_hook.py", line 18, in on_init_start
    callback.on_init_start(self)
TypeError: on_init_start() missing 1 required positional argument: 'trainer'