Restrict navigation to a single floor for 2023 challenge [DO NOT MERGE]

[srama2512]

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Motivation and Context

This change, in conjunction with this commit in habitat-lab, restricts navigation for habitat 2023 challenges to a single floor.

How Has This Been Tested

Tested locally by training ObjectNav agents.

Types of changes

• [x] New feature (non-breaking change which adds functionality)

Checklist

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