Fixes loading of material from MDL file (#268)

# Description

The Nucleus directory for materials has changed in 2023.1. They are now
directly inside `NVIDIA/Materials` instead of `NVIDIA/Assets/Materials`.
This MR fixes the referenced directory.

## Type of change

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

## Screenshots

This is how the rough terrain looks like now:

![shingle_ground](https://github.com/isaac-orbit/orbit/assets/12863862/c561f664-22bb-45d1-a713-9d95ec48feda)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] 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
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.9.54"
+version = "0.9.55"
 
 # Description
 title = "ORBIT framework for Robot Learning"

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

 Changelog
 ---------
 
+0.9.55 (2023-12-03)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the Nucleus directory path in the :attr:`omni.isaac.orbit.utils.assets.NVIDIA_NUCLEUS_DIR`.
+  Earlier, it was referring to the ``NVIDIA/Assets`` directory instead of ``NVIDIA``.
+
+
 0.9.54 (2023-11-29)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/spawners/materials/visual_materials_cfg.py

@@ -60,11 +60,12 @@ class MdlFileCfg(VisualMaterialCfg):
 
     NVIDIA Omniverse provides various MDL materials in the NVIDIA Nucleus.
     To use these materials, you can set the path of the material in the nucleus directory
-    using the ``{NVIDIA_NUCLEUS_DIR}`` variable.
+    using the ``{NVIDIA_NUCLEUS_DIR}`` variable. This is internally resolved to the path of the
+    NVIDIA Nucleus directory on the host machine through the attribute
+    :attr:`omni.isaac.orbit.utils.assets.NVIDIA_NUCLEUS_DIR`.
 
-
-    For example, to use the "Brick_Wall_Brown" material, you can set the path to:
-    ``{NVIDIA_NUCLEUS_DIR}/Materials/Brick_Wall_Brown/Brick_Wall_Brown.mdl``.
+    For example, to use the "Aluminum_Anodized" material, you can set the path to:
+    ``{NVIDIA_NUCLEUS_DIR}/Materials/Base/Metals/Aluminum_Anodized.mdl``.
     """
     project_uvw: bool | None = None
     """Whether to project the UVW coordinates of the material. Defaults to None.
@@ -76,6 +77,11 @@ class MdlFileCfg(VisualMaterialCfg):
 
     If None, then the default setting in the MDL material will be used.
     """
+    texture_scale: tuple[float, float] | None = None
+    """The scale of the texture. Defaults to None.
+
+    If None, then the default setting in the MDL material will be used.
+    """
 
 
 @configclass

source/extensions/omni.isaac.orbit/omni/isaac/orbit/utils/assets.py

@@ -32,13 +32,13 @@ if nucleus_utils.get_assets_root_path() is None:
     carb.log_error(msg)
     raise RuntimeError(msg)
 
-NVIDIA_NUCLEUS_DIR = nucleus_utils.get_nvidia_asset_root_path()
+NVIDIA_NUCLEUS_DIR = f"{nucleus_utils.get_assets_root_path()}/NVIDIA"
 """Path to the root directory on the NVIDIA Nucleus Server."""
 
 ISAAC_NUCLEUS_DIR = f"{nucleus_utils.get_assets_root_path()}/Isaac"
 """Path to the `Isaac` directory on the NVIDIA Nucleus Server."""
 
-ISAAC_ORBIT_NUCLEUS_DIR = f"{nucleus_utils.get_assets_root_path()}/Isaac/Samples/Orbit"
+ISAAC_ORBIT_NUCLEUS_DIR = f"{ISAAC_NUCLEUS_DIR}/Samples/Orbit"
 """Path to the `Isaac/Samples/Orbit` directory on the NVIDIA Nucleus Server."""
 
 

source/extensions/omni.isaac.orbit_tasks/omni/isaac/orbit_tasks/locomotion/velocity/velocity_env_cfg.py

@@ -55,6 +55,10 @@ class MySceneCfg(InteractiveSceneCfg):
             static_friction=1.0,
             dynamic_friction=1.0,
         ),
+        visual_material=sim_utils.MdlFileCfg(
+            mdl_path="{NVIDIA_NUCLEUS_DIR}/Materials/Base/Architecture/Shingles_01.mdl",
+            project_uvw=True,
+        ),
         debug_vis=False,
     )
     # robots
@@ -74,6 +78,10 @@ class MySceneCfg(InteractiveSceneCfg):
         prim_path="/World/light",
         spawn=sim_utils.DistantLightCfg(color=(0.75, 0.75, 0.75), intensity=3000.0),
     )
+    sky_light = AssetBaseCfg(
+        prim_path="/World/skyLight",
+        spawn=sim_utils.DomeLightCfg(color=(0.13, 0.13, 0.13), intensity=1000.0),
+    )
 
 
 ##