[Bugfix]--Fix HM3D Semantic Object ID construction

[jturner65]

Motivation and Context

This PR fixes how the Semantic Object ID is constructed for the HM3D dataset. Previously, it was constructing the Semantic Object ID to be "_" (i.e. "wall_1") but this was incorrect - the current standard in engine is to have the final value of the Semantic Object ID be a unique int across all categories, corresponding to the Semantic ID in the Scene Node. This PR fixes this.

How Has This Been Tested

Locally c++ and python tests pass.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[aclegg3]

Looks good to me. We should add the PoC semantic scene to the test assets download so we can verify this stuff for future proofing. We have permission to distribute that one.