Override Net::DebugString to invoke inner ops_

Net::DebugString() print what it is and invokes inner operators
DebugString() with a prefix '\t'.

paddle/framework/net.cc

@@ -55,5 +55,14 @@ void PlainNet::CompleteAddOp() {
   add_op_done_ = true;
 }
 
+std::string PlainNet::DebugString() const {
+  std::ostringstream os;
+  os << this->type_ << ":" << std::endl;
+  for (auto& op : ops_) {
+    os << "\t" << op->DebugString() << std::endl;
+  }
+  return os.str();
+}
+
 }  // namespace framework
 }  // namespace paddle

paddle/framework/net.h

@@ -88,6 +88,8 @@ class PlainNet : public Net {
 
   void CompleteAddOp() override;
 
+  std::string DebugString() const override;
+
   std::vector<OperatorPtr> ops_;
 
  private:

paddle/framework/operator.h

@@ -48,7 +48,7 @@ class OperatorBase {
     return boost::get<T>(attrs_.at(name));
   }
 
-  std::string DebugString() const;
+  virtual std::string DebugString() const;
 
   /// Init will be called after CreateOperator, you can put some initialization
   /// logic here.