add nnx_scan util module

[aireenmei]

Description

NNXDecoder._create_scanned_layers provides a generic logic for creating scanned layers. Moving it into a separate nnx_scan util module allows re-using by other modules. Use case: Gemma4 will use a custom way to implement scannable block (in an up-coming PR), and can re-use this logic. But importing from NNXDecoder will cause circular imports. A separate nnx_scan module make the code cleaner.

Also fix a bug: jnp.moveaxis(x, scan_axis, 0) -> jnp.moveaxis(x, 0, param_scan_axis), with added unit test to guard.

Tests

Unit test

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/nnx_scan.py	82.60%	3 Missing and 5 partials ⚠️

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