[IR] Reconcile the graph attribute in ir.Node

[justinchuby]

It is pointed to a graph when we call function.append(), but it is not when we call ir.Node(graph=function). Need to figure out how to reconcile this.

1. Can a node.graph point to a Function? (no)
2. The graph in Function is private, should it be exposed to users? (will expose when user calls node.graph)
3. What should users pass in to ir.Node(graph=) when the node is meant for a function being constructed? (it can be a function. function.append will do the right thing as assign the underlying graph to the node)