refactor: reorganize code and add full type hint

[XuehaiPan]

Split from #83

• Bump OpTree version to 0.2.0
• Remove directory _src
• Add full type hints

[codecov-commenter]

Codecov Report

Base: 72.72% // Head: 73.20% // Increases project coverage by +0.47% 🎉

Coverage data is based on head (e97603e) compared to base (bc7a1c5).
Patch coverage: 69.89% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   72.72%   73.20%   +0.47%     
==========================================
  Files          33       33              
  Lines        1430     1515      +85     
==========================================
+ Hits         1040     1109      +69     
- Misses        390      406      +16     

Flag	Coverage Δ
unittests	73.20% <69.89%> (+0.47%)	⬆️

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

Impacted Files	Coverage Δ
torchopt/visual.py	10.60% <20.00%> (ø)
torchopt/utils.py	28.82% <28.82%> (ø)
torchopt/transform.py	81.07% <73.80%> (ø)
torchopt/accelerated_op/__init__.py	22.72% <75.00%> (ø)
torchopt/optim/meta/base.py	62.50% <82.35%> (ø)
torchopt/linalg.py	91.78% <85.71%> (ø)
torchopt/base.py	78.26% <90.00%> (ø)
torchopt/optim/base.py	85.18% <92.00%> (ø)
torchopt/__init__.py	100.00% <100.00%> (ø)
torchopt/accelerated_op/adam_op.py	75.00% <100.00%> (ø)
... and 22 more

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