[WIP] Native tf2.x migration for AttentionOCR

[kyscg]

Description

Following up from #8843

Native TensorFlow 2 migration with some additions. Main issues to resolve are tf.contrib and tf.flags. Fixes #8703

Type of change

• 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)

Tests

To be updated in the future

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.