Add Qwen3

[bzantium]

Description

Add Qwen3 models.
Basically, the implementation is highly similar to llama2 except qk_norm so I just added use_qk_norm. Also, I added query_pre_attn_scalar which is head_dim ** 0.5 as llama4.

maxtext/MaxText/layers/qwen3.py

Lines 106 to 107 in f4a5e24

	use_qk_norm=cfg.use_qk_norm,
	query_pre_attn_scalar=query_pre_attn_scalar,

FIXES: #1834

Checklist

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• I have performed a self-review of my code.
• 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.