Properly remove in-memory references (#25652)
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Make Node::ToProto() const
call Graph::ToGraphProto() const so it does process all of the
subgraphs recursively and removes all the in-memory references.
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Node::ToProto() const does not respect constness of subgraph attributes
and calls non-const version of Graph::ToGraphProto() which does not
process
subgraph initializers and does not remove in-memory references.
