Lazily import locomotion envs to prevent ModuleNotFoundError when labmaze is not installed

[GaetanLepage]

Description

Context: bumping Shimmy version in nixpkgs

Unfortunately, we don't have labmaze packaged and this prevents shimmy from being imported at all:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<string>", line 1, in <lambda>
  File "/nix/store/zy22nlap6gznpjhqry4w0hgw5mf62q3f-python3-3.11.9/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1147, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 940, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/nix/store/q5ry6cvhi5zl8rhhw65a14xk9dal4861-python3.11-shimmy-2.0.0/lib/python3.11/site-packages/shimmy/__init__.py", line 11, in <module>
    register_gymnasium_envs()
  File "/nix/store/q5ry6cvhi5zl8rhhw65a14xk9dal4861-python3.11-shimmy-2.0.0/lib/python3.11/site-packages/shimmy/registration.py", line 180, in register_gymnasium_envs
    _register_dm_control_envs()
  File "/nix/store/q5ry6cvhi5zl8rhhw65a14xk9dal4861-python3.11-shimmy-2.0.0/lib/python3.11/site-packages/shimmy/registration.py", line 102, in _register_dm_control_envs
    from dm_control.locomotion.examples import basic_cmu_2019, basic_rodent_2020
  File "/nix/store/7aqjra9cd3hmwkx0jv81flnbrwwj8jf6-python3.11-dm-control-1.0.23/lib/python3.11/site-packages/dm_control/locomotion/examples/basic_cmu_2019.py", line 22, in <module>
    from dm_control.locomotion.arenas import corridors as corr_arenas
  File "/nix/store/7aqjra9cd3hmwkx0jv81flnbrwwj8jf6-python3.11-dm-control-1.0.23/lib/python3.11/site-packages/dm_control/locomotion/arenas/__init__.py", line 22, in <module>
    from dm_control.locomotion.arenas.labmaze_textures import FloorTextures
  File "/nix/store/7aqjra9cd3hmwkx0jv81flnbrwwj8jf6-python3.11-dm-control-1.0.23/lib/python3.11/site-packages/dm_control/locomotion/arenas/labmaze_textures.py", line 20, in <module>
    from labmaze import assets as labmaze_assets
ModuleNotFoundError: No module named 'labmaze'

This patch gracefully ignores such an ImportError and simply skips the registration of the labmaze-dependent environments in this case.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update (not sure)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Thanks for the PR

[pseudo-rnd-thoughts]

dm_control seems to be failing for reasons unrelated to your PR

[GaetanLepage]

Thanks !