NNX migration: NNX utils and unit tests

[xibinliu]

Description

• Add utils to manipulate the NNX shardings with abstract state of a model
  • also add unit tests for the utils
• Extract mesh creation function to maxtext_utils.get_mesh_from_config()
  • also add unit tests for this func

Note:
flax v0.12 has DeprecationWarning in multiple places:
- DeprecationWarning: '.value' access is now deprecated. Use variable.get_value() or variable[...] (for [Array]).
- DeprecationWarning: 'VariableState' was removed, this is just an alias to 'Variable'. Plase use 'Variable' directly instead.
But since the code needs to work with post-training, which currently requires flax v0.11, we didn't change code for these warnings.

Tests

Since there is minor change in the create_nnx_model() code, verified the SFT in single node with stable dependencies and it passed.

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 85.71429% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/maxtext_utils_nnx.py	84.84%	2 Missing and 3 partials ⚠️
src/MaxText/model_creation_utils.py	66.66%	2 Missing ⚠️

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