Use .size() instead of .Size() in download_and_convert_mnist.py

slim/datasets/download_and_convert_mnist.py

@@ -165,7 +165,7 @@ def _download_dataset(dataset_dir):
                                                _progress)
       print()
       with tf.gfile.GFile(filepath) as f:
-        size = f.Size()
+        size = f.size()
       print('Successfully downloaded', filename, size, 'bytes.')