Fix flaky test caused by weak reference

[carmocca]

What does this PR do?

Fixes:

_________________ test_dataloader_source_request_from_module __________________

    def test_dataloader_source_request_from_module():
        """Test requesting a dataloader from a module works."""
        module = BoringModel()
        module.trainer = Trainer()
        module.foo = Mock(return_value=module.train_dataloader())
    
        source = _DataLoaderSource(module, "foo")
        assert source.is_module()
        module.foo.assert_not_called()
>       assert isinstance(source.dataloader(), DataLoader)

D:\a\lightning\lightning\tests\tests_pytorch\trainer\connectors\test_data_connector.py:454: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = _DataLoaderSource(instance=BoringModel(
  (layer): Linear(in_features=32, out_features=2, bias=True)
), name='foo')

    def dataloader(self) -> Union[TRAIN_DATALOADERS, EVAL_DATALOADERS]:
        """Returns the dataloader from the source.
    
        If the source is a module, the method with the corresponding :attr:`name` gets called.
        """
        from pytorch_lightning import LightningDataModule, LightningModule  # prevent cyclic import
    
        if not self.name:
            return self.instance
    
        if isinstance(self.instance, LightningModule):
>           return self.instance.trainer._call_lightning_module_hook(self.name, pl_module=self.instance)
E           ReferenceError: weakly-referenced object no longer exists

Example: https://github.com/Lightning-AI/lightning/runs/7773962786?check_suite_focus=true#step:20:3270

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

None

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cc @Borda @akihironitta

[codecov]

Codecov Report

Merging #14157 (1259be2) into master (a7cebf2) will increase coverage by 15%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #14157     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         326      326             
  Lines       26576    26621     +45     
=========================================
+ Hits        16243    20239   +3996     
+ Misses      10333     6382   -3951     

[tchaton]

Neat !