corrected comment regarding .train and .eval (#2659)

Co-authored-by: Svetlana Karslioglu <svekars@meta.com>

beginner_source/introyt/tensorboardyt_tutorial.py

@@ -214,13 +214,14 @@ def forward(self, x):
             # Check against the validation set
             running_vloss = 0.0
 
-            net.train(False) # Don't need to track gradents for validation
+            # In evaluation mode some model specific operations can be omitted eg. dropout layer
+            net.train(False) # Switching to evaluation mode, eg. turning off regularisation
             for j, vdata in enumerate(validation_loader, 0):
                 vinputs, vlabels = vdata
                 voutputs = net(vinputs)
                 vloss = criterion(voutputs, vlabels)
                 running_vloss += vloss.item()
-            net.train(True) # Turn gradients back on for training
+            net.train(True) # Switching back to training mode, eg. turning on regularisation
 
             avg_loss = running_loss / 1000
             avg_vloss = running_vloss / len(validation_loader)