[Breaking] Set 1st dim of prediction output to be row ID

[hcho3]

When performing tree prediction in parallel, it is common to split the workload over the data row dimension. So it would be beneficial to put the row ID as the first (outer-most) dimension. In addition, both XGBoost and scikit-learn RandomForestRegressor puts the row ID in the first dimension when predicting with multi-target models.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (5b1bbff) 84.33% compared to head (c509bfa) 84.29%.

Files	Patch %	Lines
src/gtil/predict.cc	95.83%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           mainline     #549      +/-   ##
============================================
- Coverage     84.33%   84.29%   -0.04%     
============================================
  Files            71       71              
  Lines          6069     6068       -1     
  Branches        510      510              
============================================
- Hits           5118     5115       -3     
- Misses          951      953       +2     

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