[bugfix] Prevent deepcopy of dataloaders / Trainer in SWA Callback

[tchaton]

What does this PR do?

This PR introduces a context manager to prevent deecopying trainer / dataloaders from the LightningModule.
This PR uses this context manager to prevent with stochastic weight averaging
This PR improve ModelPruning save_checkpoint to avoid a deepcopy.

Fixes #7089

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

No.

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[codecov]

Codecov Report

Merging #8472 (294307c) into master (8a9ee40) will increase coverage by 0%.
The diff coverage is 100%.

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##           master   #8472   +/-   ##
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  Coverage      92%     92%           
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  Files         217     217           
  Lines       14342   14367   +25     
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+ Hits        13236   13260   +24     
- Misses       1106    1107    +1     

[carmocca]

Pushed a few commits with some changes

[kaushikb11]

LGTM!

[awaelchli]

If I understand this PR correctly, all the changes are specific to the pruning etc. callbacks. can you confirm the following:

• pickling behavior unchanged
• deepcopy(model) still does a real deepcopy unless the context manager is used
• the context manager is not meant to be used by the user, or is it? any other use cases for deepcopying a lightnignmodule?

[tchaton]

If I understand this PR correctly, all the changes are specific to the pruning etc. callbacks. can you confirm the following:

• pickling behavior unchanged
• deepcopy(model) still does a real deepcopy unless the context manager is used
• the context manager is not meant to be used by the user, or is it? any other use cases for deepcopying a lightnignmodule?

Yes, exactly !

[awaelchli]

Great!

Excluding the dataloaders here may not be necessary in the future if we avoid this weird patching of LightningModule methods: #6030