feat: add AdamW optimizer

[Benjamin-eecs]

#16

[codecov-commenter]

Codecov Report

Merging #44 (0f5c90a) into main (218d4e0) will increase coverage by 0.20%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   62.20%   62.41%   +0.20%     
==========================================
  Files          26       28       +2     
  Lines         987     1075      +88     
==========================================
+ Hits          614      671      +57     
- Misses        373      404      +31     

Flag	Coverage Δ
unittests	62.41% <66.66%> (+0.20%)	⬆️

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

Impacted Files	Coverage Δ
torchopt/_src/base.py	80.00% <ø> (ø)
torchopt/_src/hook.py	35.71% <0.00%> (ø)
torchopt/_src/optimizer/adam.py	100.00% <ø> (ø)
torchopt/_src/optimizer/meta/adam.py	87.50% <ø> (ø)
torchopt/_src/optimizer/meta/base.py	33.33% <ø> (ø)
torchopt/_src/optimizer/meta/rmsprop.py	87.50% <ø> (ø)
torchopt/_src/optimizer/meta/sgd.py	85.71% <ø> (ø)
torchopt/_src/optimizer/rmsprop.py	100.00% <ø> (ø)
torchopt/_src/optimizer/sgd.py	100.00% <ø> (ø)
torchopt/_src/transform.py	75.87% <55.55%> (-4.36%)	⬇️
... and 7 more

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