Polish code

test=develop

paddle/fluid/imperative/layer.h

@@ -136,8 +136,6 @@ class VarBase {
 
  public:
   virtual ~VarBase() {
-    // LOG(ERROR) << "remove var " << name_;
-
     if (block_ && !persistable_) {
       block_->RemoveVar(name_);
     }

python/paddle/fluid/imperative/tracer.py

@@ -24,10 +24,6 @@ __all__ = ['Tracer']
 
 
 def release_op(op):
-    import gc
-    assert len(
-        gc.get_referrers(framework._imperative_tracer()._ops[
-            op._trace_id])) == 1
     del framework._imperative_tracer()._ops[op._trace_id]
 
 
@@ -59,6 +55,8 @@ class Tracer(core.Tracer):
             if len(backward_refs) > 0:
                 op.iop.register_backward_hooks(release_op)
 
+                # TODO(minqiyang): remove all inputs and outputs after seperate
+                # var and grad
                 op.backward_refs = defaultdict(list)
                 for k, v in six.iteritems(op.inputs):
                     if k in backward_refs: