[models.densenet] Use internal model constant

src/ptbench/models/densenet.py

@@ -105,7 +105,9 @@ class Densenet(pl.LightningModule):
         self.model_ft = models.densenet121(weights=weights)
 
         # Adapt output features
-        self.model_ft.classifier = torch.nn.Linear(1024, num_classes)
+        self.model_ft.classifier = torch.nn.Linear(
+            self.model_ft.classifier.in_features, num_classes
+        )
 
     def forward(self, x):
         x = self.normalizer(x)  # type: ignore