Lazily create TrialResult.final_simulator_state

[daxfohl]

Final step of #4100, creates the TrialResult.final_simulator_state lazily so that it's possible to run an entire simulation on a sparsely entangled circuit and sample measurements without running out of memory. Unit test added with 59 qubits on density matrix simulator running 1000 repetitions (9 cirq.X's, 1 cirq.measure, on all qubits) in under 100 ms.

def test_large_untangled_okay():
    circuit = cirq.Circuit()
    for i in range(59):
        for _ in range(9):
            circuit.append(cirq.X(cirq.LineQubit(i)))
        circuit.append(cirq.measure(cirq.LineQubit(i)))
    with pytest.raises(MemoryError, match='Unable to allocate'):
        _ = cirq.DensityMatrixSimulator(split_untangled_states=False).simulate(circuit)
    result = cirq.DensityMatrixSimulator(split_untangled_states=True).simulate(circuit)
    assert set(result._step_result_or_state._qubits) == set(cirq.LineQubit.range(59))
    # _ = result.final_density_matrix hangs (as expected)
    result = cirq.DensityMatrixSimulator(split_untangled_states=True).run(circuit, repetitions=1000)
    assert len(result.measurements) == 59
    assert len(result.measurements['0']) == 1000
    assert (result.measurements['0'] == np.full(1000, 1)).all()

Supercedes #4198, which had diverged from master too much to merge.

[daxfohl]

@95-martin-orion Start this review at simulator.py; otherwise it can be confusing. Basically all I did was take the field _final_simulator_state and change it to a property that is lazily calculated from the step result, and updated the code to pass in a that step result instead of the calculated simulator state. (It also still allows the calculated state to be passed in there for backwards compatibility). Then for all subclasses that had some field based on the final state, I changed those to properties that reference the _final_simulator_state property so that they are lazily calculated too.

[95-martin-orion]

Looks good overall.

[daxfohl]

@95-martin-orion The requested changes are complete.