AutoML: stratify imbalanced datasets

[jppgks]

Adds stratified splitting to the AutoML generated config if the dataset:

• has a single output feature
• the output feature is categorical or binary
• the output feature's imbalance ratio is below imbalance_threshold

[github-actions[bot]]

Unit Test Results

         6 files  +    1         6 suites  +1   3h 36m 47s :stopwatch: + 1h 16m 17s   3 415 tests +    2  3 337 :heavy_check_mark: +    2    78 :zzz: ±  0  0 :x: ±0  10 245 runs  +140  9 988 :heavy_check_mark: +116  257 :zzz: +24  0 :x: ±0 

Results for commit 6b7f9f98. ± Comparison against base commit 1dc66cad.

:recycle: This comment has been updated with latest results.