Fixes absolute paths in instanceable meshes with URDF Converter (#225)

# Description

With at least Isaac Sim 2023.1 (not checked on 2022.2.1), the URDF
importer from Isaac Sim seems to save the material paths as absolute
when trying to save the asset in its instanceable format. The material
paths are relative when the USD is generated as non-instanceable.

This MR fixes this behavior by adding a post-processing step that
iterates over the instanced mesh file and resolves all paths to become
relative.

You can use the tool by running the following:

```bash
./orbit.sh -p source/tools/convert_urdf.py ~/git/anymal_d_simple_description/urdf/anymal.urdf source/extensions/omni.isaac.orbit_assets/data/Robots/ANYbotics/ANYmalD/anymal_instanceable.usda --merge-joints --make-instanceable
```

## Type of change

- Bug fix (non-breaking change which fixes an issue)
- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file

source/extensions/omni.isaac.orbit/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.9.28"
+version = "0.9.29"
 
 # Description
 title = "ORBIT framework for Robot Learning"

source/extensions/omni.isaac.orbit/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.9.29 (2023-11-01)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the material path resolution inside the :class:`omni.isaac.orbit.sim.converters.UrdfConverter` class.
+  With Isaac Sim 2023.1, the material paths from the importer are always saved as absolute paths. This caused
+  issues when the generated USD file was moved to a different location. The fix now resolves the material paths
+  relative to the USD file location.
+
+
 0.9.28 (2023-11-01)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/converters/asset_converter_base.py

@@ -78,7 +78,7 @@ class AssetConverterBase(abc.ABC):
             usd_file_name = cfg.usd_file_name
         # add USD extension if not provided
         if not (usd_file_name.endswith(".usd") or usd_file_name.endswith(".usda")):
-            self._usd_file_name = usd_file_name + ".usda"
+            self._usd_file_name = usd_file_name + ".usd"
         else:
             self._usd_file_name = usd_file_name
 
@@ -107,6 +107,14 @@ class AssetConverterBase(abc.ABC):
             self._convert_asset(cfg)
             # dump the configuration to a file
             dump_yaml(os.path.join(self.usd_dir, "config.yaml"), cfg.to_dict())
+            # add comment to top of the saved config file with information about the converter
+            current_date = datetime.now().strftime("%Y-%m-%d")
+            current_time = datetime.now().strftime("%H:%M:%S")
+            generation_comment = (
+                f"##\n# Generated by {self.__class__.__name__} on {current_date} at {current_time}.\n##\n"
+            )
+            with open(os.path.join(self.usd_dir, "config.yaml"), "a") as f:
+                f.write(generation_comment)
 
     """
     Properties.
@@ -135,7 +143,7 @@ class AssetConverterBase(abc.ABC):
         mesh references in the generated USD file are resolved relatively. Otherwise, it becomes
         difficult to move the USD asset to a different location.
         """
-        return os.path.join(".", "Props", "instanceable_meshes.usda")
+        return os.path.join(".", "Props", "instanceable_meshes.usd")
 
     """
     Implementation specifics.

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/converters/asset_converter_base_cfg.py

@@ -28,8 +28,11 @@ class AssetConverterBaseCfg:
     usd_file_name: str | None = None
     """The name of the generated usd file. Defaults to :obj:`None`.
 
-    If set to :obj:`None`, it is resolved from the asset file name. The extension of the asset file
-    is replaced with ``.usd``.
+    If set to :obj:`None`, it is resolved from the asset file name. For example, if the asset file
+    name is ``"my_asset.urdf"``, then the generated USD file name is ``"my_asset.usd"``.
+
+    If the providing file name does not end with ".usd" or ".usda", then the extension
+    ".usd" is appended to the file name.
     """
 
     force_usd_conversion: bool = False

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/converters/urdf_converter.py

@@ -6,9 +6,13 @@
 
 from __future__ import annotations
 
+import os
+
 import omni.kit.commands
+import omni.usd
 from omni.isaac.core.utils.extensions import enable_extension
 from omni.isaac.version import get_version
+from pxr import Usd
 
 from .asset_converter_base import AssetConverterBase
 from .urdf_converter_cfg import UrdfConverterCfg
@@ -81,6 +85,14 @@ class UrdfConverter(AssetConverterBase):
             import_config=import_config,
             dest_path=self.usd_path,
         )
+        # fix the issue that material paths are not relative
+        if self.cfg.make_instanceable:
+            usd_path = os.path.join(self.usd_dir, self.usd_instanceable_meshes_path)
+            stage = Usd.Stage.Open(usd_path)
+            # resolve all paths relative to layer path
+            source_layer = stage.GetRootLayer()
+            omni.usd.resolve_paths(source_layer.identifier, source_layer.identifier)
+            stage.Save()
 
     """
     Helper methods.

source/tools/convert_urdf.py

@@ -20,11 +20,11 @@ positional arguments:
   output              The path to store the USD file.
 
 optional arguments:
-  -h, --help            Show this help message and exit
-  --headless            Force display off at all times. (default: False)
-  --merge-joints, -m    Consolidate links that are connected by fixed joints. (default: False)
-  --fix-base, -f        Fix the base to where it is imported. (default: False)
-  --make-instanced, -i  Make the asset instanceable for efficient cloning. (default: False)
+  -h, --help                Show this help message and exit
+  --headless                Force display off at all times. (default: False)
+  --merge-joints, -m        Consolidate links that are connected by fixed joints. (default: False)
+  --fix-base, -f            Fix the base to where it is imported. (default: False)
+  --make-instanceable, -i   Make the asset instanceable for efficient cloning. (default: False)
 
 """
 
@@ -55,7 +55,7 @@ parser.add_argument(
     "--fix-base", "-f", action="store_true", default=False, help="Fix the base to where it is imported."
 )
 parser.add_argument(
-    "--make-instanced",
+    "--make-instanceable",
     "-i",
     action="store_true",
     default=False,
@@ -102,7 +102,7 @@ def main():
         fix_base=args_cli.fix_base,
         merge_fixed_joints=args_cli.merge_joints,
         force_usd_conversion=True,
-        make_instanceable=args_cli.make_instanced,
+        make_instanceable=args_cli.make_instanceable,
     )
 
     # Print info