Add unit tests for linears.py

[bvandermoon]

Description

This PR adds unit tests for src/maxtext/layers/linears.py, covering:

• Utility functions: normalize_axes, canonicalize_tuple, _convert_to_activation_function.
• DenseGeneral layer: Basic call, bias usage, and multiple axes.
• MlpBlock layer: Basic call and fused MLP configuration.
  These tests help ensure the correctness of these core layers and improve test coverage.

Tests

I tested this change by running the new unit tests on a TPU VM.
Command: pytest tests/unit/linears_test.py
Result: 8 passed

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• 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, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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