[vision][mnist] download_and_prepare should be no op if file exist.

PiperOrigin-RevId: 365275913

Author: frederick0329

official/vision/image_classification/mnist_main.py

@@ -157,7 +157,7 @@ def define_mnist_flags():
       distribution_strategy=True)
   flags_core.define_device()
   flags_core.define_distribution()
-  flags.DEFINE_bool('download', False,
+  flags.DEFINE_bool('download', True,
                     'Whether to download data to `--data_dir`.')
   flags.DEFINE_integer('profiler_port', 9012,
                        'Port to start profiler server on.')