question: Access datamodule in custom callbacks
context: I want to create a custom Callback class where I can access certain attributes from my DataModule and log/save them before the start of the train step.
  I am little confused on how to do this. Can anyone help me out with a quick snippet? Thanks!
answer: Something like this should work :]
  from pytorch_lightning.callbacks import Callback
  class MyCallback(Callback):
      def __init__(self, ...):
          ...
  
      # hook for doing something with your datamodule before training step
      def on_train_batch_start(self, trainer, *args, **kwargs):
          dm = trainer.datamodule # this is a reference to your datamodule during training
          # do something here with your datamodule