feat(treespec): add methods PyTreeSpec.entry and PyTreeSpec.child

[XuehaiPan]

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You can use the syntax close #15213 if this solves the issue #15213

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (128e912) 100.00% compared to head (0078f5b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #88   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          425       429    +4     
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+ Hits           425       429    +4     

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

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

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[JieRen98]

Why raise error for None and Leaf?

[XuehaiPan]

This is unreachable. For None and leaf, it will raise IndexError due to there are no children.

For None and leaf, root.arity = 0.

    if (index < -root.arity || index >= root.arity) [[unlikely]] {
        throw py::index_error("PyTreeSpec::Entry() index out of range.");
    }