Adds tutorial for modifying an existing Direct RL Environment (#115)

# Description

This PR adds the tutorial for Modifying an existing Direct RL
Environment.

## Type of change

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applicable. -->

- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.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
- [ ] 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

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docs/source/refs/snippets/tutorial_modify_direct_rl_env.py

+# Copyright (c) 2022-2024, The Isaac Lab Project Developers.
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+# fmt: off
+
+# [start-init-import]
+from .h1_env import H1Env, H1EnvCfg
+# [end-init-import]
+
+# [start-init-register]
+gym.register(
+    id="Isaac-H1-Direct-v0",
+    entry_point="omni.isaac.lab_tasks.direct.humanoid:H1Env",
+    disable_env_checker=True,
+    kwargs={
+        "env_cfg_entry_point": H1EnvCfg,
+        "rl_games_cfg_entry_point": f"{agents.__name__}:rl_games_ppo_cfg.yaml",
+        "rsl_rl_cfg_entry_point": f"{agents.__name__}.rsl_rl_ppo_cfg:HumanoidPPORunnerCfg",
+        "skrl_cfg_entry_point": f"{agents.__name__}:skrl_ppo_cfg.yaml",
+    },
+)
+# [end-init-register]
+
+# [start-h1_env-import]
+from omni.isaac.lab_assets import H1_CFG
+# [end-h1_env-import]
+
+# [start-h1_env-spaces]
+num_actions = 19
+num_observations = 69
+# [end-h1_env-spaces]
+
+# [start-h1_env-robot]
+robot: ArticulationCfg = H1_CFG.replace(prim_path="/World/envs/env_.*/Robot")
+joint_gears: list = [
+    50.0,  # left_hip_yaw
+    50.0,  # right_hip_yaw
+    50.0,  # torso
+    50.0,  # left_hip_roll
+    50.0,  # right_hip_roll
+    50.0,  # left_shoulder_pitch
+    50.0,  # right_shoulder_pitch
+    50.0,  # left_hip_pitch
+    50.0,  # right_hip_pitch
+    50.0,  # left_shoulder_roll
+    50.0,  # right_shoulder_roll
+    50.0,  # left_knee
+    50.0,  # right_knee
+    50.0,  # left_shoulder_yaw
+    50.0,  # right_shoulder_yaw
+    50.0,  # left_ankle
+    50.0,  # right_ankle
+    50.0,  # left_elbow
+    50.0,  # right_elbow
+]
+# [end-h1_env-robot]
+
+# fmt: on

docs/source/tutorials/03_envs/index.rst

@@ -15,3 +15,4 @@ different aspects of the framework to create a simulation environment for agent
     create_direct_rl_env
     register_rl_env_gym
     run_rl_training
+    modify_direct_rl_env

docs/source/tutorials/03_envs/modify_direct_rl_env.rst

+.. _tutorial-modify-direct-rl-env:
+
+
+Modifying an existing Direct RL Environment
+===========================================
+
+.. currentmodule:: omni.isaac.lab
+
+Having learnt how to create a task in :ref:`tutorial-create-direct-rl-env`, register it in :ref:`tutorial-register-rl-env-gym`,
+and train it in :ref:`tutorial-run-rl-training`, we will now look at how to make minor modifications to an existing task.
+
+Sometimes it is necessary to create, due to complexity or variations from existing examples, tasks from scratch. However, in certain situations,
+it is possible to start from the existing code and introduce minor changes, one by one, to transform them according to our needs.
+
+In this tutorial, we will make minor modifications to the direct workflow Humanoid task to change the simple
+humanoid model to the Unitree H1 humanoid robot without affecting the original code.
+
+
+The Base Code
+~~~~~~~~~~~~~
+
+For this tutorial, we start from the direct workflow Humanoid environment defined in ``omni.isaac.lab_tasks.direct.humanoid`` module.
+
+.. dropdown:: Code for humanoid_env.py
+   :icon: code
+
+   .. literalinclude:: ../../../../source/extensions/omni.isaac.lab_tasks/omni/isaac/lab_tasks/direct/humanoid/humanoid_env.py
+      :language: python
+      :linenos:
+
+
+The Changes Explained
+~~~~~~~~~~~~~~~~~~~~~
+
+Duplicating the file and registering a new task
+-----------------------------------------------
+
+To avoid modifying the code of the existing task, we will make a copy of the file containing the Python
+code and perform the modification on this copy. Then, in the Isaac Lab project
+``source/extensions/omni.isaac.lab_tasks/omni/isaac/lab_tasks/direct/humanoid``
+folder we make a copy of the ``humanoid_env.py`` file and rename it to ``h1_env.py``.
+
+Open the ``h1_env.py`` file in a code editor and replace all the humanoid task name (``HumanoidEnv``) and its configuration
+(``HumanoidEnvCfg``) instances to ``H1Env`` and ``H1EnvCfg`` respectively.
+This is necessary to avoid name conflicts during import when registering the environment.
+
+Once the name change has been made, we proceed to add a new entry to register the task under the name ``Isaac-H1-Direct-v0``.
+To do this, we modify the ``__init__.py`` file in the same working folder and add the following entry.
+Refer to the :ref:`tutorial-register-rl-env-gym` tutorial for more details about environment registrations.
+
+.. hint::
+
+   If the changes in the task are minimal, it is very likely that the same RL library agent configurations can be used to train it successfully.
+   Otherwise, it is advisable to create new configuration files (adjusting their name during registration under the ``kwargs`` parameter)
+   to avoid altering the original configurations.
+
+
+.. literalinclude:: ../../refs/snippets/tutorial_modify_direct_rl_env.py
+   :language: python
+   :start-after: [start-init-import]
+   :end-before: [end-init-import]
+
+.. literalinclude:: ../../refs/snippets/tutorial_modify_direct_rl_env.py
+   :language: python
+   :start-after: [start-init-register]
+   :end-before: [end-init-register]
+
+Changing the robot
+------------------
+
+The ``H1EnvCfg`` class (in the new created ``h1_env.py`` file) encapsulates the configuration values of the environment,
+including the assets to be instantiated. Particularly in this example, the ``robot`` property holds the target articulation configuration.
+
+Since the Unitree H1 robot is included in the Isaac Lab assets extension (``omni.isaac.lab_assets``) we can just import it
+and do the replacement directly (under the ``H1EnvCfg.robot`` property), as shown below. Note that we also need to modify the
+``joint_gears`` property as it holds robot-specific configuration values.
+
+.. |franka-direct-link| replace:: `Isaac-Franka-Cabinet-Direct-v0 <https://github.com/isaac-sim/IsaacLab/blob/main/source/extensions/omni.isaac.lab_tasks/omni/isaac/lab_tasks/direct/franka_cabinet/franka_cabinet_env.py>`__
+
+.. hint::
+
+   If the target robot is not included in the Isaac Lab assets extension, it is possible to load and configure it, from a USD file,
+   by using the :class:`~omni.isaac.lab.assets.ArticulationCfg` class.
+
+   * See the |franka-direct-link| source code for an example of loading and configuring a robot from a USD file.
+   * Refer to the `Importing a New Asset <../../how-to/import_new_asset.html>`_ tutorial for details on how to import an asset from URDF or MJCF file, and other formats.
+
+.. literalinclude:: ../../refs/snippets/tutorial_modify_direct_rl_env.py
+   :language: python
+   :start-after: [start-h1_env-import]
+   :end-before: [end-h1_env-import]
+
+.. literalinclude:: ../../refs/snippets/tutorial_modify_direct_rl_env.py
+   :language: python
+   :start-after: [start-h1_env-robot]
+   :end-before: [end-h1_env-robot]
+
+The robot changed, and with it the number of joints to control or the number of rigid bodies that compose the articulation, for example.
+Therefore, it is also necessary to adjust other values in the environment configuration that depend on the characteristics of the robot,
+such as the number of elements in the observation and action space.
+
+.. literalinclude:: ../../refs/snippets/tutorial_modify_direct_rl_env.py
+   :language: python
+   :start-after: [start-h1_env-spaces]
+   :end-before: [end-h1_env-spaces]
+
+The Code Execution
+~~~~~~~~~~~~~~~~~~
+
+After the minor modification has been done, and similar to the previous tutorial, we can train on the task using one of the available RL workflows for such task.
+
+.. code-block:: bash
+
+  ./isaaclab.sh -p source/standalone/workflows/rl_games/train.py --task Isaac-H1-Direct-v0 --headless
+
+When the training is finished, we can visualize the result with the following command.
+To stop the simulation, you can either close the window, or press ``Ctrl+C`` in the terminal
+where you started the simulation.
+
+.. code-block:: bash
+
+  ./isaaclab.sh -p source/standalone/workflows/rl_games/play.py --task Isaac-H1-Direct-v0 --num_envs 64
+
+.. figure:: ../../_static/tutorials/tutorial_modify_direct_rl_env.jpg
+    :align: center
+    :figwidth: 100%
+    :alt: result of training Isaac-H1-Direct-v0 task
+
+In this tutorial, we learnt how to make minor modifications to an existing environment without affecting the original code.
+
+It is important to note, however, that while the changes to be made may be small, they may not always work on the first try,
+as there may be deeper dependencies on the original assets in the environment being modified.
+In these cases, it is advisable to analyze the code of the available examples in detail in order to make an appropriate adjustment.

docs/source/tutorials/03_envs/register_rl_env_gym.rst

+.. _tutorial-register-rl-env-gym:
+
 Registering an Environment
 ==========================
 

docs/source/tutorials/03_envs/run_rl_training.rst

+.. _tutorial-run-rl-training:
+
 Training with an RL Agent
 =========================