Enable gradients at train start (#2200)

* Enable gradients at train start

* Update training_loop.py

Co-authored-by: William Falcon <waf2107@columbia.edu>

pytorch_lightning/trainer/training_loop.py

@@ -319,6 +319,12 @@ class TrainerTrainLoopMixin(ABC):
         # get model
         model = self.get_model()
 
+        # enable train mode
+        model.train()
+
+        # enable gradients
+        torch.set_grad_enabled(True)
+
         # load data
         # if reload_dataloaders_every_epoch, this is moved to the epoch loop
         if not self.reload_dataloaders_every_epoch: