I don't quite understand the phrase "managed reference". Could anyone explain
it to me? Really appreciate your help! 
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Topic](https://discuss.tvm.apache.org/t/questions-about-ir-expressions-relayexpr-relay
Hello. I'm reading C++ source code and quite confused by two classes related to
RelayIR - RelayExpr and RelayExprNode. I'm wondering what's the relationship
between these two types? Since RelayExpr can be cast to RelayExprNode* by the
method "get()", and RelayExprNode can be cast to RelayExpr
