Allow copying in the format cupy_array[:] = numpy_array
Allow copying in the format cupy_array[:] = numpy_array

The __setitem__ implementation from cupy.ndarray checks for an empty slice and
if the value being passed is an instance of numpy.ndarray to make a copy of it.
That can is a very useful feature in circumstances where we want to create a
copy of a numpy.ndarray without requiring mechanisms to identify the type of an
array as a CuPy array.

cupy/cupy,cupy/cupy,cupy/cupy,cupy/cupy