Mypy complaining about `transfer_batch_to_device` as abstract method

[gianscarpe]

🐛 Bug

In my projects, mypy complains if I don't override transfer_batch_to_device in DataModule. This is probably due to a design choice, since transfer_batch_to_device is an abstract method. However, it's clear that overriding the method is not mandatory, as the same behavior is obtained (probably somewhere else in lightiningmodule and trainer code, I couldn't find it), and therefore I think it shouldn't be abstractmethod
I believe we can design this a little better, by making clear that it's not mandatory to override transfer_batch_to_device

To Reproduce

Simply implement a subclass of DataModule and use mypy to check your code

Expected behavior

I expect to

Environment

• PyTorch Version (e.g., 1.7): `.7
• OS (e.g., Linux): LINUX
• How you installed PyTorch (conda, pip, source): pip
• Build command you used (if compiling from source):
• Python version: 3.8.4
• CUDA/cuDNN version: 11.0
• GPU models and configuration:
• Any other relevant information:

Additional context