fix(ops): align kwargs for tree_reduce with functools.reduce with the Python C implementation

[XuehaiPan]

Description

Describe your changes in detail.

The argument in tree_reduce was changed from initial to initializer in #39. I realized this is a documentation issue in Python. The signature in Python C code is initial. This PR reverts the changes for the argument name in tree_reduce.

Ref:

• Function signature mismatch for functools.reduce between C implementation, Python implementation, and online documentaion python/cpython#102757
• gh-102757: fix function signature mismatch for functools.reduce between code and documentation python/cpython#102759

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

• I have raised an issue to propose this change (required for new features and bug fixes)

• Function signature mismatch for functools.reduce between C implementation, Python implementation, and online documentaion python/cpython#102757

• gh-102757: fix function signature mismatch for functools.reduce between code and documentation python/cpython#102759

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)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (68a0143) 100.00% compared to head (8215d39) 100.00%.

❗ Current head 8215d39 differs from pull request most recent head 24d53e8. Consider uploading reports for the commit 24d53e8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          424       424           
=========================================
  Hits           424       424           

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

Flags with carried forward coverage won't be shown. Click here to find out more.

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

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[Benjamin-eecs]

LGTM!