fix dump callback

tensorpack/callbacks/base.py

@@ -30,7 +30,6 @@ class Callback(object):
     def before_train(self, trainer):
         self.trainer = trainer
         self.graph = tf.get_default_graph()
-        self.sess = tf.get_default_session()
         self.epoch_num = 0
         self._before_train()
 

tensorpack/callbacks/dump.py

@@ -32,7 +32,7 @@ class DumpParamAsImage(Callback):
         self.var = self.graph.get_tensor_by_name(self.var_name)
 
     def _trigger_epoch(self):
-        val = self.sess.run(self.var)
+        val = self.trainer.sess.run(self.var)
         if self.func is not None:
             val = self.func(val)
         if isinstance(val, list):