Fix assert wandb Run when mode="disabled" (#14112)

Author: gautierdag

src/pytorch_lightning/loggers/wandb.py

@@ -328,7 +328,7 @@ def __getstate__(self) -> Dict[str, Any]:
 
     @property  # type: ignore[misc]
     @rank_zero_experiment
-    def experiment(self) -> Run:
+    def experiment(self) -> Union[Run, RunDisabled]:
         r"""
 
         Actual wandb object. To use wandb features in your
@@ -361,11 +361,13 @@ def experiment(self) -> Run:
                 self._experiment = wandb.init(**self._wandb_init)
 
                 # define default x-axis
-                if isinstance(self._experiment, Run) and getattr(self._experiment, "define_metric", None):
+                if isinstance(self._experiment, (Run, RunDisabled)) and getattr(
+                    self._experiment, "define_metric", None
+                ):
                     self._experiment.define_metric("trainer/global_step")
                     self._experiment.define_metric("*", step_metric="trainer/global_step", step_sync=True)
 
-        assert isinstance(self._experiment, Run)
+        assert isinstance(self._experiment, (Run, RunDisabled))
         return self._experiment
 
     def watch(self, model: nn.Module, log: str = "gradients", log_freq: int = 100, log_graph: bool = True) -> None: