Sobel Gradients Operator

[bhashemian]

Fixes #4997

Description

Implements Sobel gradients operators for a given size of kernel.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

It looks good to me, thanks!

[bhashemian]

@wyli @Nic-Ma please let me know if you have any comment here. Thanks

[wyli]

thanks, I thought sobel kernel is always 3x3, why is there a size parameter, and it's unclear to me what sobel gradient is...perhaps a reference here?

[bhashemian]

thanks, I thought sobel kernel is always 3x3, why is there a size parameter, and it's unclear to me what sobel gradient is...perhaps a reference here?

You are right, Sobel kernels are 3x3 but they can be generalized to a larger kernel size with different approaches. You can find a nice writeup on how to derive larger size kernels following the same property of 3x3 Sobel kernels: https://stackoverflow.com/a/41065243

Following the original implementation of HoVerNet, we are going to use 5x5 Sobel kernels in HoVerNet pipeline, and they have implemented kernels similar to this PR. Here is their implementation:
https://github.com/vqdang/hover_net/blob/842964dc5d26ffe196d236d16fad16643a103850/models/hovernet/utils.py#L124-L145
https://github.com/vqdang/hover_net/blob/842964dc5d26ffe196d236d16fad16643a103850/models/hovernet/utils.py#L150

[wyli]

/build