Initialize collective_graph_key based on the graph if unspecified in RunOptions.

Before this CL, for collective_ops to work, the client had to specify a
collective_graph_key in the RunOptions of a session.Run call.

After this change, if a client does not specify a collective_graph_key for a
graph that contains collective ops, a graph key is generated automatically as a
hash of the set of keys of collective instances in the placed graph.

PiperOrigin-RevId: 211024617