[Named Topologies] Add nodes_to_linequbits method to LineTopology

[tanujkhattar]

Routing utilities currently present in cirq contrib expect a networkx graph of device (/target topology) where each node is a cirq.Qid. NamedTopology stores topology graphs where each node is a number / tuple.
TiltedSquareLattice has a nodes_to_gridqubits method which can provide a mapping from the graph nodes to the corresponding grid qubits.

Cirq/cirq-core/cirq/devices/named_topologies.py

Line 233 in d42e058

def nodes_to_gridqubits(self, offset=(0, 0)) -> Dict[Tuple[int, int], 'cirq.GridQubit']:

This can be used to quickly get the graph needed for routing circuits from the topology graph as follows:

routing_graph = nx.relabel_nodes(topology.graph, topology.nodes_to_gridqubits())

A similar convenience method, nodes_to_linequbits, should be added to the LineQubit named topology as well.

cc @mpharrigan

[mpharrigan]

Yeah, I thought about this and almost included it in #4628 but omitted it because I figured it would be less useful for people putting stuff on gridqubit-based devices.

It would be

return {i: q for i, q in enumerate(LineQubit.range(self.n)}