Fix cuRobo planner example startup

[yuecideng]

Description

This PR fixes the standalone cuRobo planner example startup by defaulting num_envs to one when no Gym configuration merge occurs. It also builds the simulation in headless mode first and opens the interactive viewer only after the robot and obstacles are loaded and placed at their final initial poses.

This prevents the example from raising a TypeError when --num_envs is omitted and avoids opening the viewer before scene setup is complete.

Dependencies: None.

Issue: N/A (reported directly; no linked GitHub issue).

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

Not applicable.

Testing

• black .
• black --check --diff --color ./
• pytest tests/sim/planners/test_curobo_planner.py (31 passed, 3 skipped)
• pytest tests (772 passed, 103 skipped)
• Interactive cuRobo example run with delayed viewer startup, forward planning, trajectory replay, and return planning

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.