Generalizes Qwen3's partial RoPE implementation into a reusable PartialRotaryEmbedding layer.

[gagika]

Description

Generalizes Qwen3's partial RoPE implementation into a reusable PartialRotaryEmbedding layer.

Previously, partial RoPE was implemented specifically for Qwen3 as Qwen3NextRotaryEmbedding. This PR refactors it to be model-agnostic to adopt partial RoPE across other architectures.

Tests

• Unit Tests: Added a dedicated unit test suite at tests/unit/partial_rotary_embedding_test.py that verifies:
  • partial_rotary_factor=0.5 correctly rotates the first half of the hidden dimension and passes the second half through unmodified.
  • partial_rotary_factor=1.0 behaves identically to the base RotaryEmbedding.
  • Shift invariance is maintained.

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.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/attentions.py	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

🤖 Hi @gagika, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

🤖 I'm sorry @gagika, but I was unable to process your request. Please see the logs for more details.

[shuningjin]

LGTM

[parambole]

LGTM