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pytorch_lightning/trainer/trainer.py

@@ -1412,6 +1412,7 @@ def call_hook(
             if hook_name in ("on_train_start",) and hasattr(self.accelerator, hook_name):
                 accelerator_hook = getattr(self.accelerator, hook_name)
                 accelerator_output = accelerator_hook(*args, **kwargs)
+                # Rely on the accelerator output if lightningModule hook returns nothing
                 # Required for cases such as DataParallel where we reduce the output for the user
                 # todo: move this data parallel logic into the data parallel plugin
                 output = accelerator_output if output is None else output