style: miscellaneous style housekeeping

[XuehaiPan]

Description

Describe your changes in detail.

Split from #87

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)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• 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

All modified lines are covered by tests ✅

Comparison is base (c1da9d0) 100.00% compared to head (a1e4e29) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #92   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          429       431    +2     
=========================================
+ Hits           429       431    +2     

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

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

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

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[Benjamin-eecs]

LGTM!