Use isinstance in internal comparison for splitter, create a splitter example

[mfeurer]

See this on how the comparison is done based on the classname: https://github.com/automl/auto-sklearn/blob/master/autosklearn/evaluation/train_evaluator.py#L913

Instead, we should use isinstance

[mfeurer]

This would also solve #498 when using sklearn's PredefinedSplitter.

[github-actions]

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