Add habitat_hitl.disable_agents_and_stepping config param

[eundersander]

Motivation and Context

This is handy if you just want to load scenes but not run policies or otherwise simulate agents. Because this codepath bypasses all policy initialization and inference code, it's also potentially a way to test a HITL app on lower-power machines.

To use this feature, enable it through the Hydra config. You could edit the config yaml, or add a command-line argument, e.g.:
python examples/hitl/basic_viewer/basic_viewer.py habitat_hitl.disable_policies_and_stepping=True

How Has This Been Tested

Local testing on Macbook.

Types of changes

• [HITL framework]

Checklist

• My code follows the code style of this project.
• I have updated the documentation if required.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes if required.

[henrysamer]

Thanks for the functionality. Please, modify the "Motivation" section Readme command line:

From

python examples/hitl/basic_viewer/basic_viewer.py habitat_hitl.disable_agents_and_stepping=True

to:

python examples/hitl/basic_viewer/basic_viewer.py +habitat_hitl.disable_agents_and_stepping=True