Fixed AttributeError in training_apg.ipynb for newer MJX versions

[ankitmalakar7]

Fixes #2822

Problem

The notebook mjx/training_apg.ipynb fails with AttributeError: 'State' object has no attribute 'pos' when using MuJoCo MJX 3.3.5+. The issue occurs because the MJX Data class no longer exposes .x and .xd attributes directly.

Solution

Added a helper function _extract_x_xd() that extracts Transform (x) and Motion (xd) objects from MJX data using the underlying arrays (data.xpos, data.xquat, data.cvel) that are available in all MJX versions.

Testing

• No syntax errors in notebook
• Compatible with both old (3.3.0) and new (3.3.5+) MJX versions
• Uses the same logic as existing mjx_to_brax() method

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