Support per sample topk loss for semantic segmentation

[srihari-humbarwadi]

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation update
• [ ] TensorFlow 2 migration
• [x] 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

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