Fix neighbouring cell gradient contamination in _extend_centers_gpu

[remi-pigache]

Fixes #1486
By using the already defined isneighbor array, the 'grad' values concerning pixels outside of the cell can be filtered out. It effectively makes the gradient generation independant of whether the pixels outside of the cell are background or belonging to another cell. Please find attached a single 3D slice of the input masks, the flows before and after the fix and the masks before and after the fix using the following script :

from cellpose.dynamics import masks_to_flows_gpu_3d, follow_flows, get_masks_torch
import tifffile
import numpy as np
masks = tifffile.imread("masks.tif")
inds = np.nonzero(masks>0)
flows = masks_to_flows_gpu_3d(masks)
p = follow_flows(flows, inds)
regenerated_masks = get_masks_torch(p.int(), inds, masks.shape)
tifffile.imwrite("masks_regenerated.tif", regenerated_masks)
tifffile.imwrite("flows.tif", flows)

input image

current ZYX flows | current masks reconstruction

fixed ZYX flows | fixed masks reconstruction