The hardcoded placements. This provides a placement for each supported
cirq.NamedTopology. The topology serves as the key for the mapping dictionary.
Each placement is a dictionary mapping topology node to final cirq.Qid device
qubit.
topo_node_to_qubit_func
A function that maps from cirq.NamedTopology nodes
to cirq.Qid. There is a correspondence between nodes and the "abstract" Qids
used to construct the un-placed circuit. We use this function to interpret
the provided mappings. By default: nodes which are tuples correspond
to cirq.GridQubits; otherwise cirq.LineQubit.