fix gemma4 dtype mismatch

[aireenmei]

Description

When fixing another bug in #3727 , I changed to use config.weight_dtype from config.dtype to initialize layer_scalar because it's a weight, this requires casting layer_scalar back to dtype during use time, or it causes error The input carry component c[1] has type bfloat16[2048,4096,2816] but the corresponding output carry component has type float32[2048,4096,2816], so the dtypes do not match. when weight_dtype=float32, dtype=bfloat16

Tests

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/models/gemma4.py	0.00%	1 Missing ⚠️

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[shralex]

typo in title dtye

[gagika]

nit: self.layer_scalar.value.astype(cfg.dtype) probably matches better maxtext style.