feat(treespec): add method PyTreeSpec.one_level

[XuehaiPan]

Description

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Motivation and Context

Why is this change required? What problem does it solve?
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You can use the syntax close #15213 if this solves the issue #15213

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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)

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• I have updated the documentation accordingly.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (111ffa1) to head (452c1a0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #179   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         3789      3801   +12     
=========================================
+ Hits          3789      3801   +12     

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

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