points default data_dir to child folder. (#6131)

Failed test is python2  and was a kokoro failure

official/resnet/cifar10_main.py

@@ -234,7 +234,7 @@ def cifar10_model_fn(features, labels, mode, params):
 def define_cifar_flags():
   resnet_run_loop.define_resnet_flags()
   flags.adopt_module_key_flags(resnet_run_loop)
-  flags_core.set_defaults(data_dir='/tmp/cifar10_data',
+  flags_core.set_defaults(data_dir='/tmp/cifar10_data/cifar-10-batches-bin',
                           model_dir='/tmp/cifar10_model',
                           resnet_size='56',
                           train_epochs=182,