Consider upgrading checkpoint files

[ejm714]

As of version 1.9, when Lightning loads a checkpoint, it now applies a version migration on-the-fly, but it does not modify your checkpoint file.

First, we should determine if this has any performance implications. If so, we should upgrade our checkpoint files permanently following these instructions: https://pytorch-lightning.readthedocs.io/en/stable/common/checkpointing_migration.html

Changelog is here