Add multi-asset loading with spacing in preview script

[yuecideng]

Description

This PR adds multi-asset support to the asset preview script.

It allows passing multiple paths to --asset_path and loads each asset with configurable relative spacing (default 1.0m) along +X from the provided initial position.

Dependencies: None

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

N/A

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.

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Pull request overview

Adds support for loading multiple assets in the preview_asset.py script by allowing multiple --asset_path values and spacing each asset along +X from the initial position.

Changes:

• Rename single-asset loader to load_assets() and return a list of loaded assets.
• Add --asset_spacing CLI argument and place assets at init_pos + (idx * spacing, 0, 0).
• Update preview/main flow to work with multiple loaded assets.

Comments suppressed due to low confidence (1)

embodichain/lab/scripts/preview_asset.py:76

• The load_assets() docstring claims that when --asset_type is not specified and the file is USD, the script will inspect the USD stage to auto-select between articulation vs rigid. In the current implementation asset_type is always taken from args.asset_type (defaulting to "rigid") except for the URDF override, so no USD auto-detection occurs. Please either implement the described USD inspection (likely requiring --asset_type defaulting to None so you can detect whether the user provided it) or update the docstring/CLI help to reflect the actual behavior.

    If ``--asset_type`` is not specified and the file is USD, the script will
    inspect the USD stage for articulation roots to decide between articulation
    and rigid object.  For non-USD files the default is always ``rigid``.

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