Encourage resetting all validation metrics when training starts (#540)

src/models/mnist_module.py

@@ -56,7 +56,9 @@ def forward(self, x: torch.Tensor):
 
     def on_train_start(self):
         # by default lightning executes validation step sanity checks before training starts,
-        # so we need to make sure val_acc_best doesn't store accuracy from these checks
+        # so it's worth to make sure validation metrics don't store results from these checks
+        self.val_loss.reset()
+        self.val_acc.reset()
         self.val_acc_best.reset()
 
     def model_step(self, batch: Any):