Add gradient for slice function

[erikabor]

I added the gradient for the slice function in tensorflow / haskell. Please check if the implementation is right. I used the version in python for template, see array_grad.py.

The CLA is signed by the manager of my company, Active Group GmbH. Please let me know if more info is needed (same as with pull requests by @rikvdkleij).

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[blackgnezdo]

Thanks for your contribution! I believe you need to troubleshoot the CLA with your company's signer:

Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot Public version.

I did confirm that Active Group GmbH has a corporate CLA signed, so it's just your association with the CLA that's missing. Since you are saying @rikvdkleij already had it work, maybe figure out how your setup is different from his?

[erikabor]

My POC added me to the group of authorized contributors.

[googlebot]

CLAs look good, thanks!

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[blackgnezdo]

@fkm3, do you think you could take a look?

[fkm3]

Thanks for the PR!

[fkm3]

Could you sync this CL to resolve the merge conflicts and then I'll submit it? Thanks!

[erikabor]

Issues should be resolved.

Comments were addressed.