Fix evaluation mistake

1 should not shuffle during evaluation; 2 num_readers should be one, or each sample would be evaluate for (num_reader) times and only (num_examples / num_reader) distinct samples are evaluted.

object_detection/samples/configs/ssd_mobilenet_v1_pets.config

@@ -183,4 +183,6 @@ eval_input_reader: {
     input_path: "PATH_TO_BE_CONFIGURED/pet_val.record"
   }
   label_map_path: "PATH_TO_BE_CONFIGURED/pet_label_map.pbtxt"
+  shuffle: false
+  num_readers: 1
 }