[trainer] Hardcode name of loss to monitor

src/mednet/engine/trainer.py

@@ -93,8 +93,6 @@ def run(
         main_pid=os.getpid(),
     )
 
-    monitor_key = "loss/validation"
-
     # This checkpointer will operate at the end of every validation epoch
     # (which happens at each checkpoint period), it will then save the lowest
     # validation loss model observed.  It will also save the last trained model
@@ -102,7 +100,7 @@ def run(
         dirpath=output_folder,
         filename=CHECKPOINT_ALIASES["best"],
         save_last=True,  # will (re)create the last trained model, at every iteration
-        monitor=monitor_key,
+        monitor="loss/validation",
         mode="min",
         save_on_train_epoch_end=True,
         every_n_epochs=validation_period,  # frequency at which it checks the "monitor"