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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.