Remove deprecated LightningModule.on_post_move_to_device

[akihironitta]

What does this PR do?

Part of #12521.

resolves #9525

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

Removes LightningModule.on_post_move_to_device.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @kaushikb11 @rohitgr7 @Borda @carmocca @justusschock @awaelchli @ananthsub @ninginthecloud @jjenniferdai

[kaushikb11]

LGTM!

[andreimargeloiu]

Is there a replacement for LightningModule.on_post_move_to_device ?

[carmocca]

@andreimargeloiu this hook was misleading as it was only called when using the tpu spawn strategy. It's likely different from what you are looking for. But to answer you, the next hook to run would be on_fit_start

If you want a hook related to moving a batch to device, you can use one of on_before_batch_transfer, transfer_batch_to_device, on_after_batch_transfer