feat(treespec): expose node kind enum for PyTreeSpec

[XuehaiPan]

Description

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Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

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• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
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• I have ensured make test pass. (required)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1982314) 100.00% compared to head (a049d11) 100.00%.

❗ Current head a049d11 differs from pull request most recent head 37a4a29. Consider uploading reports for the commit 37a4a29 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #98   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          517       515    -2     
=========================================
- Hits           517       515    -2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files	Coverage Δ
optree/__init__.py	100.00% <ø> (ø)
optree/registry.py	100.00% <100.00%> (ø)

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