Inaccurate type 1 transform in Python about cufinufft

[MelodyShih]

I found that the error is caused by this line plan.set_pts(to_gpu(kx), to_gpu(ky)).
The GPUArrays return by to_gpu(kx), to_gpu(ky) seem to be freed earlier than we want.

Changing the related lines as follows fixes the error.

kx_gpu = to_gpu(kx)
ky_gpu = to_gpu(ky)
c_gpu = to_gpu(c)

plan = cufinufft(1, (N1, N2), dtype=dtype)
plan.set_pts(kx_gpu, ky_gpu)

I am not sure why there is no problem of running the example codes example2d1many.py and example2d2many.py. But it seems that it is not from the small M we used here, e.g. changing n_transf from 2 to 1 also gives order 1 error.

I will create a pull request to update the example codes.