Add TODO for memory optimization in python3 and polishing code

python/paddle/fluid/io.py

@@ -606,6 +606,7 @@ def save_inference_model(dirname,
         feeded_var_names = [feeded_var_names.encode()]
     else:
         if len(feeded_var_names) > 0:
+            # TODO(paddle-dev): polish these code blocks
             if not (bool(feeded_var_names) and all(
                     isinstance(name, six.binary_type)
                     for name in feeded_var_names)):

python/paddle/fluid/transpiler/distribute_transpiler.py

@@ -1245,6 +1245,8 @@ class DistributeTranspiler(object):
 
     def _is_splited_grad_var(self, var, var_dict):
         grad_block = None
+        # TODO(minqiyang): replace these items() with six.iteritems() to
+        # improve memory
         for _, g in list(var_dict.items()):
             if self._orig_varname(g.name) == self._orig_varname(var.name):
                 if g.name.find(".trainer_") == -1: