Docs on sanity_checking property (#17579)

(cherry picked from commit ba14069)

docs/source-pytorch/common/trainer.rst

@@ -1360,6 +1360,18 @@ both conditions are met. If any of these arguments is not set, it won't be consi
     model = LitModel()
     trainer.fit(model)
 
+sanity_checking
+***************
+
+Indicates if the trainer is currently running sanity checking. This property can be useful to disable some hooks,
+logging or callbacks during the sanity checking.
+
+.. code-block:: python
+
+    def validation_step(self, batch, batch_idx):
+        ...
+        if not self.trainer.sanity_checking:
+            self.log("value", value)
 
 num_training_batches
 ********************

src/lightning/pytorch/trainer/trainer.py

@@ -1361,6 +1361,10 @@ def evaluating(self) -> bool:
 
     @property
     def sanity_checking(self) -> bool:
+        """Whether sanity checking is running.
+
+        Useful to disable some hooks, logging or callbacks during the sanity checking.
+        """
         return self.state.stage == RunningStage.SANITY_CHECKING
 
     @sanity_checking.setter