doc changes re at-functor and at-layer (#2390)
* doc changes re at-functor and at-layer
* fix a doctest
* more fixes
* public at-layer
* add a sentence comparing to freeze/thaw
* Apply suggestions from code review
Co-authored-by: Kyle Daruwalla <daruwalla@wisc.edu>
* two fixes re SignDecay
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Co-authored-by: Kyle Daruwalla <daruwalla@wisc.edu>
