Sobel gradient calculation transform

[bhashemian]

HoVerNet losses requires calculation of numerical gradient of HoVerMaps in horizontal and vertical direction. This can be achieved by a Sobel kernel. skimage implementation uses 3x3 Sobel kernel but this pipeline requires a 5x5 kernel. Thus having a Sobel filter to calculate gradients with a flexible kernel size would be very helpful.

[wyli]

there's a relevant transform

MONAI/monai/transforms/post/array.py

Line 560 in 99b9dd7

kernel = torch.tensor([[-1, -1, -1], [-1, 8, -1], [-1, -1, -1]], dtype=torch.float32)

and utility

MONAI/monai/networks/layers/simplelayers.py

Line 245 in 99b9dd7

def apply_filter(x: torch.Tensor, kernel: torch.Tensor, **kwargs) -> torch.Tensor: