Clear all parameters' gradient

test=develop

paddle/fluid/imperative/layer.h

@@ -152,12 +152,14 @@ class VarBase {
 
   void ClearGradient() {
     VLOG(1) << "clear gradient of " << var_desc_->Name();
+    if (grads_ && grads_->var_ && grads_->var_->IsInitialized()) {
       auto grads_t = grads_->var_->GetMutable<framework::LoDTensor>();
       operators::math::set_constant(
           *(platform::DeviceContextPool::Instance().Get(
               grads_->var_->Get<framework::LoDTensor>().place())),
           grads_t, 0.0);
     }
+  }
 
   framework::LoDTensor& GradValue();
 

python/paddle/fluid/imperative/layers.py

@@ -52,7 +52,6 @@ class Layer(core.Layer):
 
     def clear_gradients(self):
         for p in self.parameters():
-            if not p._stop_gradient:
             p._clear_gradient()
 
     def _build_once(self, inputs):