Expose viewer module when importing mujoco

[WT-MM]

(base) wesleymaa@MacBook-Pro ~ % python
Python 3.11.11 | packaged by conda-forge | (main, Dec  5 2024, 14:21:42) [Clang 18.1.8 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import mujoco
>>> mujoco.viewer
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'mujoco' has no attribute 'viewer'
>>> 

The mujoco.viewer module is not re-imported, so code that tries to call it will fail. This is obscured on MacOS because using mjpython seems to add the module to path already or otherwise import it.

This PR re-imports mujoco.viewer

(base) wesleymaa@MacBook-Pro ~ % mjpython
Python 3.11.11 | packaged by conda-forge | (main, Dec  5 2024, 14:21:42) [Clang 18.1.8 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import mujoco
>>> mujoco.viewer
<module 'mujoco.viewer' from '/Users/wesleymaa/.conda/envs/ksim/lib/python3.11/site-packages/mujoco/viewer.py'>

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[yuvaltassa]

Thanks! Can you please do the CLA thing? (Update: sorry! you did that 🙂 )