[bugfix] remove nan loss in manual optimization (#5121)

* remove nan loss whe missing

* Update pytorch_lightning/core/lightning.py

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

* Apply suggestions from code review

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>
Co-authored-by: Rohit Gupta <rohitgr1998@gmail.com>

pytorch_lightning/core/lightning.py

@@ -1393,12 +1393,15 @@ def get_progress_bar_dict(self):
         """
         # call .item() only once but store elements without graphs
         running_train_loss = self.trainer.train_loop.running_loss.mean()
-        avg_training_loss = (
-            running_train_loss.cpu().item()
-            if running_train_loss is not None
-            else float("NaN")
-        )
-        tqdm_dict = {"loss": "{:.3g}".format(avg_training_loss)}
+        avg_training_loss = None
+        if running_train_loss is not None:
+            avg_training_loss = running_train_loss.cpu().item()
+        elif self.trainer.train_loop.automatic_optimization:
+            avg_training_loss = float('NaN')
+
+        tqdm_dict = {}
+        if avg_training_loss is not None:
+            tqdm_dict["loss"] = f"{avg_training_loss:.3g}"
 
         if self.trainer.truncated_bptt_steps is not None:
             tqdm_dict["split_idx"] = self.trainer.split_idx