Fix NNX ValueError in NNXDecoder

[charlesli640]

Description

To fix the NNX ValueError
ValueError: Cannot assign data value of type '<class 'maxtext.layers.nnx_wrappers.ToNNX'>' to static attribute 'decoder' of Pytree type '<class 'maxtext.models.models.Transformer'>'.

Tests

Run distill_train

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/models/models.py	0.00%	2 Missing ⚠️

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