Add LightningDataModule.load_from_checkpoint to load datamodules directly from checkpoint

[rohitgr7]

What does this PR do?

Fixes #12062

Does your PR introduce any breaking changes? If yes, please list them.

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cc @carmocca @awaelchli @Borda @ananthsub @ninginthecloud @jjenniferdai @rohitgr7

[awaelchli]

This is a nice feature addition!

I have one concern question where I believe we should look for a decision before merging the PR.
In #10375 I propose to deprecate support for saving and restoring from a singular hparams container passed as input argument. If we decide to do this deprecation in this release, it would be better if the implementation here would already adopt the new restricted (simpler) way of parsing the args. If you think this is less convenient, then I'm also good with this. I just want to avoid us introducing a new feature with a broader scope and immediately deprecate partial functionality.

cc @carmocca

[rohitgr7]

This is a nice feature addition!

I have one concern question where I believe we should look for a decision before merging the PR. In #10375 I propose to deprecate support for saving and restoring from a singular hparams container passed as input argument. If we decide to do this deprecation in this release, it would be better if the implementation here would already adopt the new restricted (simpler) way of parsing the args. If you think this is less convenient, then I'm also good with this. I just want to avoid us introducing a new feature with a broader scope and immediately deprecate partial functionality.

cc @carmocca

let's do it in this release in a follow-up. I have seen similar issues related to it on slack so would be worth restricting the containers here.

[Borda]

lgtm

[Borda]

.