Replace opaque return types in optim

[benbaarber]

Pull Request Template

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

#1755

Changes

Updated Adam, AdamW, and AdaGrad config init to return OptimizerAdaptor<O<B::InnerBackend>, M, B> instead of the opaque type impl Optimizer<M, B>. Now the result of OptimizerConfig.init() can be stored in structs, solving the main problem in the above linked issue.

Testing

No additional testing required, just function signature changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.61%. Comparing base (e4d0cf3) to head (8c841b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1767   +/-   ##
=======================================
  Coverage   86.61%   86.61%           
=======================================
  Files         700      700           
  Lines       83473    83479    +6     
=======================================
+ Hits        72301    72307    +6     
  Misses      11172    11172           

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