Move MaxText Kernels

[dipannita08]

Description

This PR moves the kernels modules into the restructured location.
Details here

Tests

• Relevant unit tests
• Manual tests which exercise moved modules.
• Presubmits

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 33.33333% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...c/maxtext/kernels/attention/jax_flash_attention.py	5.55%	17 Missing ⚠️
src/maxtext/kernels/megablox/ops.py	50.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[khatwanimohit]

LGTM