enhance interpolate op when there is no "scale" (#1957)

test=develop

lite/operators/interpolate_op.cc

@@ -79,9 +79,15 @@ bool InterpolateOp::AttachImpl(const cpp::OpDesc& op_desc, lite::Scope* scope) {
   auto Out = op_desc.Output("Out").front();
   param_.X = scope->FindVar(X)->GetMutable<lite::Tensor>();
   param_.Out = scope->FindVar(Out)->GetMutable<lite::Tensor>();
-  param_.scale = op_desc.GetAttr<float>("scale");
-  param_.out_w = op_desc.GetAttr<int>("out_w");
-  param_.out_h = op_desc.GetAttr<int>("out_h");
+  if (op_desc.HasAttr("scale")) {
+    param_.scale = op_desc.GetAttr<float>("scale");
+  }
+  if (op_desc.HasAttr("out_w")) {
+    param_.out_w = op_desc.GetAttr<int>("out_w");
+  }
+  if (op_desc.HasAttr("out_h")) {
+    param_.out_h = op_desc.GetAttr<int>("out_h");
+  }
   param_.align_corners = op_desc.GetAttr<bool>("align_corners");
   param_.interp_method = op_desc.GetAttr<std::string>("interp_method");
   return true;