enable on_load_checkpoint for datamodule for all trainer_fn

[rohitgr7]

What does this PR do?

Fixes #10229
In the past, datamodule states were loaded only with .fit while resuming the training using Trainer(resume_from_checkpoint=...). But now since we allow checkpoint for other trainer methods, and datamodule contains all the data-related info, it seems reasonable to load its states as well during .test/predict/validate. The PR changes this behavior.

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[tchaton]

Neat !

[tchaton]

LGTM !