Fix memory leak in custatevec

[dlucchetti]

Example stack-trace:

multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/lib/python3.12/multiprocessing/pool.py", line 125, in worker
result = (True, func(*args, **kwds))
^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/multiprocessing/pool.py", line 48, in mapstar
return list(map(*args))
^^^^^^^^^^^^^^^^
File "/gpu/lib/python3.12/site-packages/qemulator/job_wrappers/run_multisim.py", line 108, in multiproc_sim_wrapper
return run_sim(kwargs), run_info
^^^^^^^^^^^^^^^^^
File "/gpu/lib/python3.12/site-packages/qemulator/job_wrappers/run_sim.py", line 307, in run_sim
state = state.reset()
^^^^^^^^^^^^^
File "/gpu/lib/python3.12/site-packages/pecos/simulators/custatevec/state.py", line 103, in reset
self.cupy_vector = cp.zeros(shape=2self.num_qubits, dtype=self.cp_type)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/gpu/lib/python3.12/site-packages/cupy/_creation/basic.py", line 250, in zeros
a = cupy.ndarray(shape, dtype, order=order)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "cupy/_core/core.pyx", line 151, in cupy._core.core.ndarray.new
File "cupy/_core/core.pyx", line 239, in cupy._core.core._ndarray_base._init
File "cupy/cuda/memory.pyx", line 738, in cupy.cuda.memory.alloc
File "cupy/cuda/memory.pyx", line 1424, in cupy.cuda.memory.MemoryPool.malloc
File "cupy/cuda/memory.pyx", line 1445, in cupy.cuda.memory.MemoryPool.malloc
File "cupy/cuda/memory.pyx", line 1116, in cupy.cuda.memory.SingleDeviceMemoryPool.malloc
File "cupy/cuda/memory.pyx", line 1137, in cupy.cuda.memory.SingleDeviceMemoryPool._malloc
File "cupy/cuda/memory.pyx", line 1382, in cupy.cuda.memory.SingleDeviceMemoryPool._try_malloc
File "cupy/cuda/memory.pyx", line 1385, in cupy.cuda.memory.SingleDeviceMemoryPool._try_malloc
cupy.cuda.memory.OutOfMemoryError: Out of memory allocating 68,719,476,736 bytes (allocated so far: 68,719,476,736 bytes).
"""

[ciaranra]

LGTM