fixed epoch is None - it is not None

solver.py

@@ -274,7 +274,7 @@ class Solver():
             epoch (int): Current epoch value
         """
 
-        if epoch is None:
+        if epoch is not None:
             checkpoint_file_path = os.path.join(
                 self.experiment_directory_path, checkpoint_directory, 'checkpoint_epoch_' + str(epoch) + '.' + checkpoint_extension)
             self._checkpoint_reader(checkpoint_file_path)