Remove Duplicate Splits During Training

[JinLi711]

When finding the best splits, sometimes we may be trying the same split more than once.

This happens because:

• say we have the categories [a, b, c]. Currently, our algorithm considers both splits [a], [b, c] and [b, c], [a], even when they are the same split.
• when randomly sampling during the choice of splits, we may randomly sample the same splits more than once.