Add Qwen3 Audio Encoder

[aireenmei]

Description

Original author @eitanporat in #2726

Revise according to the original PR comments:

• rename flags for consistency and clean up usage
• make AudioEncoder an NNX module
• remove SinusoidsPositionEmbedding, replaced with expanded existing PositionalEmbedding module
• remove Qwen3OmniAudioModel in qwen3.py since we use the AudioEncoder class in decoder.py instead
• add precision related flags

Tests

tests/check_qwen3_omni_audio_vs_reference.py all pass

Checklist

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• 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 19.65812% with 282 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/multimodal_utils.py	12.63%	155 Missing and 4 partials ⚠️
src/MaxText/layers/qwen3.py	26.88%	68 Missing ⚠️
src/MaxText/layers/encoders.py	19.23%	21 Missing ⚠️
src/MaxText/layers/models.py	23.52%	11 Missing and 2 partials ⚠️
src/MaxText/layers/embeddings.py	31.25%	11 Missing ⚠️
src/MaxText/layers/decoders.py	0.00%	4 Missing and 1 partial ⚠️
src/MaxText/maxtext_utils.py	62.50%	2 Missing and 1 partial ⚠️
src/MaxText/multimodal/utils.py	0.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[hengtaoguo]

Thank you for the great work!

[NicoGrande]

LGTM