sdf2usd: nested models are not supported

[scpeters]

Environment

• OS Version: macOS 14.7 (Sonoma)
• Source or binary build? source build of main branch (178e136) with Gazebo Ionic

Description

• Expected behavior: converting a file with nested models should work properly
• Actual behavior: the conversion fails with an error message (see src/sdf_parser/Model.cc:57-63

Steps to reproduce

1. Build gz-usd
2. Download the nested_model_joint_positions.sdf example world into the gz-usd build folder
3. From the gz-usd build folder, convert to USD with the following command:

./bin/sdf2usd nested_model_joint_positions.sdf nested_model_joint_positions.usda

Output

$ ./bin/sdf2usd nested_model_joint_positions.sdf nested_model_joint_positions.usda
The following errors occurred when parsing world [nested_model_joint_positions]:
Error Code 2: Msg: Error parsing model [model2]
Error Code 18: Msg: Nested models currently aren't supported.
Error Code 2: Msg: Error parsing model [model3]
Error Code 18: Msg: Nested models currently aren't supported.
Error Code 2: Msg: Error parsing model [model4]
Error Code 18: Msg: Nested models currently aren't supported.