fix stratified sampling call

Proposed changes

Fixes https://github.com/uber/causalml/issues/755 (never actually using stratified sampling because of an incorrect call to scikit-learn's train_test_split).

Types of changes

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[x] Bugfix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation Update (if none of the other choices apply)

Checklist

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uber / causalml