DeepSeek V4 Integration

[parambole]

Description

Start with a short description of what the PR does and how this is a change from
the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456
FIXES: #123456

Notice 1: Once all tests pass, the "pull ready" label will automatically be assigned.
This label is used for administrative purposes. Please do not add it manually.

Notice 2: For external contributions, our settings currently require an approval from a MaxText maintainer to trigger CI tests.

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• 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

❌ Patch coverage is 13.59223% with 356 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/attention_compressed.py	8.48%	205 Missing ⚠️
src/maxtext/models/deepseek_v4.py	31.76%	58 Missing ⚠️
src/maxtext/layers/moe.py	12.96%	47 Missing ⚠️
src/maxtext/layers/nnx_decoders.py	2.77%	35 Missing ⚠️
src/maxtext/layers/mhc.py	11.11%	8 Missing ⚠️
src/maxtext/layers/decoders.py	0.00%	2 Missing ⚠️
src/maxtext/layers/normalizations.py	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!