Log total_loss during training

Summary: This diff adds the sum of all the losses as `total_loss` and logs it.

Reviewed By: kazhang

Differential Revision: D31063260

fbshipit-source-id: 3012dd49dd8f5fc60a7c32f3ad7a3477d2b6f5a0

d2go/runner/lightning_task.py

@@ -168,6 +168,7 @@ class DefaultTask(pl.LightningModule):
     def training_step(self, batch, batch_idx):
         loss_dict = self.forward(batch)
         losses = sum(loss_dict.values())
+        loss_dict["total_loss"] = losses
         self.storage.step()
 
         self.log_dict(loss_dict, prog_bar=True)