Add rrule for DiagonalTensorMap and include tests

[lkdvos]

Implement the rrule for DiagonalTensorMap(::AbstractTensorMap) and add corresponding tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.66%. Comparing base (8e3af86) to head (02e8a15).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
+ Coverage   82.64%   82.66%   +0.02%     
==========================================
  Files          43       43              
  Lines        5559     5568       +9     
==========================================
+ Hits         4594     4603       +9     
  Misses        965      965              

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