Split plot refinement (accurate labeling)

[tbohne]

I used get_splits(train_labels, valid_size=.2, stratify=True, random_state=23, shuffle=True).
In this case, I would expect the second label to be "Valid" instead of "Test". I'm not specifying any test split and by default it is zero. I am, however, specifying a valid_size, which is why the labels should be "Train" and "Validation", not "Test", but it looks like this:

I made a small change to plot_splits() to change the behavior to my needs. For some reason, it assumed that one split, i.e., two lists, always means (train, test). I realized that validation data is not optional in the split generation function, so I assumed it as mandatory. So the combination of only "Train" and "Test" is not possible.

Behavior now:

• specify valid_size and test_size -> three lists and labels "Train", "Valid" and "Test":
• 
• specify valid_size -> two lists and labels "Train" and "Valid":
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• specify test_size -> three lists and labels "Train", "Valid" and "Test" (since valid data is mandatory):
• 
• specify test_size and set valid_size to 0 -> two lists and labels "Valid" and "Test" (in this case valid == train):
• 

This is a reasonable labeling behavior in my opinion (under the assumption that validation data is mandatory).
I also set a default value for the new parameter in plot_splits() so that it doesn't cause any compatibility issues.

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