Support teardown for Lightning DataModule

[ananthsub]

🚀 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

[stale]

This issue has been automatically marked as stale because it hasn't had any recent activity. This issue will be closed in 7 days if no further activity occurs. Thank you for your contributions, Pytorch Lightning Team!