Support teardown for Lightning DataModule
🚀 Feature
teardown as a hook can be useful for data modules.
Motivation
This could be used for:

Clean up downloaded data after training finishes
Closing any open connections a dataloader makes
etc

Pitch
This has natural connections to prepare_data and setup and could be implemented very similarly to how those are supported across the data module and lightning module.
By default this should do nothing
cc @nateraw