Check graph outputs are defined

[gramalingam]

The current checker does not check that all graph outputs are defined. This PR adds such a check. (See issue #6040 ).

Note: We probably also want a separate check to ensure that a graph-input is not used as a graph-output. But leaving that for a separate PR, since it is a distinctly different check (and perhaps might require a different discussion).

Redo of PR #6070

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.05%. Comparing base (83194ed) to head (9b4ca5e).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6083      +/-   ##
==========================================
+ Coverage   56.95%   57.05%   +0.09%     
==========================================
  Files         506      506              
  Lines       30467    30489      +22     
  Branches     4592     4593       +1     
==========================================
+ Hits        17353    17394      +41     
+ Misses      12285    12269      -16     
+ Partials      829      826       -3     

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