Fix use of non-common dataset interface in generic models

The common dataset interface exposes a get_mean_file() method to retrieve the mean file.
Using the dataset.mean_file attribute only works for "images/generic" datasets.
This should have been changed in NVIDIA#723

digits/model/images/generic/job.py

@@ -44,10 +44,10 @@ def download_files(self, epoch=-1):
         download_files = [(self.path(filename), os.path.basename(filename))
                           for filename in model_files.values()]
 
-        if task.dataset.mean_file:
+        if task.dataset.get_mean_file():
             download_files.append((
-                task.dataset.path(task.dataset.mean_file),
-                os.path.basename(task.dataset.mean_file)))
+                task.dataset.path(task.dataset.get_mean_file()),
+                os.path.basename(task.dataset.get_mean_file())))
 
         # add snapshot
         download_files.append((snapshot_filename,