Add support for un-instantiated inner optimizers

[odow]

Closes #176

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.26%. Comparing base (b7594e2) to head (88d87f5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   99.19%   99.26%   +0.06%     
==========================================
  Files           5        5              
  Lines        1488     1492       +4     
==========================================
+ Hits         1476     1481       +5     
+ Misses         12       11       -1     

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