add warning message to failed pop.

Author: luomai

tensorlayer/_logging.py

@@ -5,3 +5,7 @@
 
 def info(fmt, *args):
     logging.info(fmt, *args)
+
+
+def warning(fmt, *args):
+    logging.warning(fmt, *args)

tensorlayer/layers/recurrent.py

@@ -148,7 +148,7 @@ def __init__(
             try:
                 cell_init_args.pop('state_is_tuple')
             except Exception:
-                pass
+                logging.warning('pop state_is_tuple fails.')
 
         self.inputs = layer.outputs