Add unit tests for embeddings.py and RoPE variants

[bvandermoon]

Description

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

• Embed class: Basic call and attend method.
• RotaryEmbedding class: Basic call.
• RoPE variants: PartialRotaryEmbedding, Gemma4PartialRotaryEmbedding, LLaMARotaryEmbedding, and YarnRotaryEmbedding.
• Added snapshot assertions for RoPE variants to verify output values against current implementation.
  These tests help ensure the correctness of these complex embedding layers and protect against future regressions.

Tests

I tested this change by running the new unit tests on a TPU VM.
Command: pytest tests/unit/embeddings_test.py
Result: 7 passed

Checklist

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• 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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