Provide a way to avoid copying state vector from Cirq

[mhucka]

What is your request or suggestion?

An internal user reported getting the following error with a 40 qubit simulation:

Traceback (most recent call last):
  File "/tmp/test/exact_sim.py", line 23, in <module>
    psi = qsim.simulate(circuit, qubit_order=qubits).state_vector()
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/venv/lib64/python3.12/site-packages/cirq/sim/simulator.py", line 496, in simulate
    return self.simulate_sweep(
           ^^^^^^^^^^^^^^^^^^^^
  File "/tmp/venv/lib64/python3.12/site-packages/cirq/sim/simulator.py", line 511, in simulate_sweep
    return list(self.simulate_sweep_iter(program, params, qubit_order, initial_state))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/venv/lib64/python3.12/site-packages/qsimcirq/qsim_simulator.py", line 560, in simulate_sweep_iter
    final_state = cirq.StateVectorSimulationState(
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/venv/lib64/python3.12/site-packages/cirq/sim/state_vector_simulation_state.py", line 351, in __init__
    state = _BufferedStateVector.create(
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/venv/lib64/python3.12/site-packages/cirq/sim/state_vector_simulation_state.py", line 84, in create
    state_vector = state_vector.copy()
                   ^^^^^^^^^^^^^^^^^^^
numpy._core._exceptions._ArrayMemoryError: Unable to allocate 8.00 TiB for an array with shape (2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2) and data type complex64

They were able to work around the problem by calling QSimSimulator()._simulate_impl() directly.

What solution or approach do you envision?

We should investigate whether the copy can be handled in a more user-friendly way.

How urgent is this for you?

P2 – needed within two quarters