Update base.py (#224)

This is a lazy solution to #223

Author: icarosadero

cebra/solver/base.py

@@ -210,7 +210,8 @@ def fit(
                         self.decoding(loader, valid_loader))
                 if save_hook is not None:
                     save_hook(num_steps, self)
-                self.save(logdir, f"checkpoint_{num_steps:#07d}.pth")
+                if logdir is not None:
+                    self.save(logdir, f"checkpoint_{num_steps:#07d}.pth")
 
     def step(self, batch: cebra.data.Batch) -> dict:
         """Perform a single gradient update.