Inherit from Callback throws missing Trainer argument
🐛 Bug

The PL 1.0.x update has removed the trainer argument from TrainerCallbackHookMixin when calling e.g. callback.on_init_start(self). However, Callback has not been updated for this change. This results in a missing argument trainer.

TrainerCallbackHookMixin link: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/trainer/callback_hook.py#L42
Callback link: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/callbacks/base.py#L38

Please reproduce using the BoringModel and post here

BoringModel colab: https://colab.research.google.com/drive/1i38oUFbnkgmgamMk6Jc4zpc9BrF31ZfE?usp=sharing
To Reproduce

BoringModel
Expected behavior

Not throw an error when using class MyPrintingCallback(Callback)
Actual behavior
TypeError                                 Traceback (most recent call last)

<ipython-input-13-1f9f6fbe4f6c> in <module>()
----> 1 test_x(tmpdir)

3 frames

/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/callback_hook.py in on_init_start(self)
     40         """Called when the trainer initialization begins, model has not yet been set."""
     41         for callback in self.callbacks:
---> 42             callback.on_init_start(self)
     43 
     44     def on_init_end(self):

TypeError: on_init_start() missing 1 required positional argument: 'trainer'
Environment

PyTorch Version (e.g., 1.0): 1.0.2
OS (e.g., Linux): Google Colab

Additional context