feat(diff/zero_order): add OOP API for zero-order differentiation

[XuehaiPan]

Description

Describe your changes in detail.

Add OOP API for zero-order differentiation.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• 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)
• Example (update in the folder of example)

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-commenter]

Codecov Report

Base: 69.52% // Head: 70.08% // Increases project coverage by +0.55% 🎉

Coverage data is based on head (dbe9c1b) compared to base (03686dc).
Patch coverage: 94.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   69.52%   70.08%   +0.55%     
==========================================
  Files          69       71       +2     
  Lines        2901     2948      +47     
==========================================
+ Hits         2017     2066      +49     
+ Misses        884      882       -2     

Flag	Coverage Δ
unittests	70.08% <94.00%> (+0.55%)	⬆️

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

Impacted Files	Coverage Δ
torchopt/diff/zero_order/nn/module.py	91.89% <91.89%> (ø)
torchopt/diff/__init__.py	100.00% <100.00%> (ø)
torchopt/diff/implicit/nn/__init__.py	100.00% <100.00%> (ø)
torchopt/diff/zero_order/__init__.py	100.00% <100.00%> (+9.09%)	⬆️
torchopt/diff/zero_order/nn/__init__.py	100.00% <100.00%> (ø)
torchopt/nn/__init__.py	100.00% <100.00%> (ø)
torchopt/diff/zero_order/decorator.py	96.81% <0.00%> (+1.81%)	⬆️

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