Fix bug in metric tracker

[SkafteNicki]

What does this PR do?

Fixes #1304
Wrong variable was returned in the return_step=False case due to confusing variable naming.

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[codecov]

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[stancld]

Our integration tests stared failing:

C:\hostedtoolcache\windows\Python\3.10.8\x64\lib\site-packages\pytorch_lightning\trainer\trainer.py:579: in fit
    call._call_and_handle_interrupt(
C:\hostedtoolcache\windows\Python\3.10.8\x64\lib\site-packages\pytorch_lightning\trainer\call.py:38: in _call_and_handle_interrupt
    return trainer_fn(*args, **kwargs)
C:\hostedtoolcache\windows\Python\3.10.8\x64\lib\site-packages\pytorch_lightning\trainer\trainer.py:621: in _fit_impl
    self._run(model, ckpt_path=self.ckpt_path)
C:\hostedtoolcache\windows\Python\3.10.8\x64\lib\site-packages\pytorch_lightning\trainer\trainer.py:984: in _run
    verify_loop_configurations(self)
C:\hostedtoolcache\windows\Python\3.10.8\x64\lib\site-packages\pytorch_lightning\trainer\configuration_validator.py:55: in verify_loop_configurations
    _check_on_epoch_start_end(model)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

model = TestModel(
  (layer): Linear(in_features=32, out_features=2, bias=True)
  (metric_step): SumMetric()
  (metric_epoch): SumMetric()
)

    def _check_on_epoch_start_end(model: "pl.LightningModule") -> None:
        hooks = (
            ("on_epoch_start", "on_<train/validation/test>_epoch_start"),
            ("on_epoch_end", "on_<train/validation/test>_epoch_end"),
        )
    
        for hook, alternative_hook in hooks:
            if callable(getattr(model, hook, None)):
>               raise RuntimeError(
                    f"The `LightningModule.{hook}` hook was removed in v1.8. Please use"
                    f" `LightningModule.{alternative_hook}` instead."
                )
E               RuntimeError: The `LightningModule.on_epoch_start` hook was removed in v1.8. Please use `LightningModule.on_<train/validation/test>_epoch_start` instead.

@Borda @justusschock

---> #1308