PR #21365: Fix nadam optimizer

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Imported from GitHub PR #21365

Resolves issues #15035 and #13980.

Copybara import of the project:

  - 4d389e6cee2e0b355ee27a529864c8843438487e fix nadam optimizer by Emma Strubell <strubell@cs.umass.edu>
  - 1f2816b3063a6a1c635db48cc52604acbbc63682 update nadam optimizer test and fix nadam to work with Re... by Emma Strubell <strubell@cs.umass.edu>
  - 3110cb6d13a24bb0434f4bc4944931fc0cd27d16 fix whitespace and elaborate on comment by Emma Strubell <strubell@cs.umass.edu>
  - 98deea7f207b226a152509dd663af6192147bb30 actually fix whitespace by Emma Strubell <strubell@cs.umass.edu>
  - 42292b7a430460b30d12bc4c03b30eeeec1e9900 fix line length by Emma Strubell <strubell@cs.umass.edu>
  - e05efc155344b9aec22b037894d92eb4a65be4e6 Merge 42292b7a430460b30d12bc4c03b30eeeec1e9900 into a767a... by Emma Strubell <emma.strubell@gmail.com>

PiperOrigin-RevId: 221850946