Support per sample topk loss for semantic segmentation

[srihari-humbarwadi]

• Bug fix (non-breaking change which fixes an issue)
• Documentation update
• TensorFlow 2 migration
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• A new research paper code implementation
• Other (Specify)

Checklist

• I have signed the Contributor License Agreement.
• I have read guidelines for pull request.
• My code follows the coding guidelines.
• I have performed a self code review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.

[fyangf]

@arashwan Can you take a look?

[laxmareddyp]

Hi @srihari-humbarwadi,

The beta folder has been deprecated.

In future, please open an issue and discuss with us before submitting a pull request. If the change does not align with our best practices, we can provide early feedback. Please review guidelines here: https://github.com/tensorflow/models/wiki/Submitting-a-pull-request

Thanks.