Migrate Qwen3 Next to Checkpoint Util

[Rohan-Bierneni]

Description

Qwen3 Next currenty has checkpoint scripts for scanned and unscanned from HF to Maxtext. This pr onboards the model to the checkpoint util which allows for bidirectional conversion between HF and MaxText.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456

Tests

Will run forward pass logit checker on converted checkpoint from Maxtext -> HF -> Maxtext for scanned and post results here:

Current status:

• Forward logit pass (hf -> maxtext scanned): https://paste.googleplex.com/5151438898593792

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.