Explicit config_class instead of module inspection

src/transformers/modeling_tf_bert.py

@@ -473,6 +473,8 @@ class TFBertNSPHead(tf.keras.layers.Layer):
 
 @keras_serializable
 class TFBertMainLayer(tf.keras.layers.Layer):
+    config_class = BertConfig
+
     def __init__(self, config, **kwargs):
         super().__init__(**kwargs)
         self.num_hidden_layers = config.num_hidden_layers