Warn users when using multi-dim MultiDiscrete obs space (DLR-RM#2003)

* Update env checker to warn users when using multi-dim MultiDiscrete obs space

* Update changelog

Author: araffin

docs/misc/changelog.rst

@@ -3,7 +3,7 @@
 Changelog
 ==========
 
-Release 2.4.0a8 (WIP)
+Release 2.4.0a9 (WIP)
 --------------------------
 
 .. note::
@@ -13,13 +13,21 @@ Release 2.4.0a8 (WIP)
   To suppress the warning, simply save the model again.
   You can find more info in `PR #1963 <https://github.com/DLR-RM/stable-baselines3/pull/1963>`_
 
+.. warning::
+
+    Stable-Baselines3 (SB3) v2.4.0 will be the last one supporting Python 3.8 (end of life in October 2024)
+    and PyTorch < 2.0.
+    We highly recommended you to upgrade to Python >= 3.9 and PyTorch >= 2.0.
+
+
 Breaking Changes:
 ^^^^^^^^^^^^^^^^^
 
 New Features:
 ^^^^^^^^^^^^^
 - Added support for ``pre_linear_modules`` and ``post_linear_modules`` in ``create_mlp`` (useful for adding normalization layers, like in DroQ or CrossQ)
 - Enabled np.ndarray logging for TensorBoardOutputFormat as histogram (see GH#1634) (@iwishwasaneagle)
+- Updated env checker to warn users when using multi-dim array to define `MultiDiscrete` spaces
 
 Bug Fixes:
 ^^^^^^^^^^

stable_baselines3/common/env_checker.py

@@ -98,6 +98,14 @@ def _check_unsupported_spaces(env: gym.Env, observation_space: spaces.Space, act
                 "is not supported but `dict(space2=Box(), spaces3=Box(), spaces4=Discrete())` is."
             )
 
+    if isinstance(observation_space, spaces.MultiDiscrete) and len(observation_space.nvec.shape) > 1:
+        warnings.warn(
+            f"The MultiDiscrete observation space uses a multidimensional array {observation_space.nvec} "
+            "which is currently not supported by Stable-Baselines3. "
+            "Please convert it to a 1D array using a wrapper: "
+            "https://github.com/DLR-RM/stable-baselines3/issues/1836."
+        )
+
     if isinstance(observation_space, spaces.Tuple):
         warnings.warn(
             "The observation space is a Tuple, "

stable_baselines3/version.txt

@@ -1 +1 @@
-2.4.0a8
+2.4.0a9

tests/test_envs.py

@@ -123,6 +123,8 @@ def patched_step(_action):
         spaces.Dict({"img": spaces.Box(low=0, high=255, shape=(32, 32, 3), dtype=np.uint8)}),
         # Non zero start index
         spaces.Discrete(3, start=-1),
+        # 2D MultiDiscrete
+        spaces.MultiDiscrete(np.array([[4, 4], [2, 3]])),
         # Non zero start index (MultiDiscrete)
         spaces.MultiDiscrete([4, 4], start=[1, 0]),
         # Non zero start index inside a Dict