Clarify output dimensions of GTIL

[hcho3]

• Document the expected dimensions of the prediction.
• Always use the first dimension to represent output targets, even for single target models.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (96e0c48) 84.29% compared to head (da9b60d) 84.33%.

Additional details and impacted files

@@             Coverage Diff              @@
##           mainline     #548      +/-   ##
============================================
+ Coverage     84.29%   84.33%   +0.03%     
============================================
  Files            71       71              
  Lines          6069     6069              
  Branches        510      510              
============================================
+ Hits           5116     5118       +2     
+ Misses          953      951       -2     

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