fix: CometLogger now correctly identifies when to create new Experiment object

Author: Etay Livne

src/lightning/pytorch/loggers/comet.py

@@ -268,7 +268,7 @@ def experiment(self) -> Union["Experiment", "ExistingExperiment", "OfflineExperi
             self.logger.experiment.some_comet_function()
 
         """
-        if self._experiment is not None:
+        if self._experiment is not None and self.experiment.alive:
             return self._experiment
 
         if self._future_experiment_key is not None: