fix multi-process reader not exist in resume (#3323)

tools/train.py

@@ -103,10 +103,6 @@ def main():
             optimizer = optim_builder(lr)
             optimizer.minimize(loss)
 
-    train_reader = create_reader(train_feed, cfg.max_iters * devices_num,
-                                 FLAGS.dataset_dir)
-    train_pyreader.decorate_sample_list_generator(train_reader, place)
-
     # parse train fetches
     train_keys, train_values, _ = parse_fetches(train_fetches)
     train_values.append(lr)
@@ -163,6 +159,13 @@ def main():
     elif cfg.pretrain_weights:
         checkpoint.load_pretrain(exe, train_prog, cfg.pretrain_weights)
 
+    train_reader = create_reader(
+                    train_feed, 
+                    (cfg.max_iters - start_iter) * devices_num,
+                    FLAGS.dataset_dir)
+    train_pyreader.decorate_sample_list_generator(train_reader, place)
+
+
     # whether output bbox is normalized in model output layer
     is_bbox_normalized = False
     if hasattr(model, 'is_bbox_normalized') and \