move train_batch_size sync from load_config to merge_config (#1510)

* move train_batch_size sync from load_config to merge_config

ppdet/core/workspace.py

@@ -98,14 +98,6 @@ def load_config(file_path):
 
     merge_config(cfg)
 
-    # NOTE: training batch size defined only in TrainReader, sychornized
-    #       batch size config to global, models can get batch size config
-    #       from global config when building model.
-    #       batch size in evaluation or inference can also be added here
-    if 'TrainReader' in global_config:
-        global_config['train_batch_size'] = global_config['TrainReader'][
-            'batch_size']
-
     return global_config
 
 
@@ -141,7 +133,16 @@ def merge_config(config, another_cfg=None):
     """
     global global_config
     dct = another_cfg if another_cfg is not None else global_config
-    return dict_merge(dct, config)
+    dct = dict_merge(dct, config)
+
+    # NOTE: training batch size defined only in TrainReader, sychornized
+    #       batch size config to global, models can get batch size config
+    #       from global config when building model.
+    #       batch size in evaluation or inference can also be added here
+    if 'TrainReader' in dct:
+        dct['train_batch_size'] = dct['TrainReader']['batch_size']
+
+    return dct
 
 
 def get_registered_modules():