Performs validation of alternative evaluations.

extracts PCollection of extracts.
alternatives Dict of PTransforms (Extracts -> Evaluation) whose output will be compared for validation purposes (e.g. 'baseline' vs 'candidate').
validators List of validators for validating the output from running the alternatives. The Validation outputs produced by the validators will be merged into a single output. If there are overlapping output keys, later outputs will replace earlier outputs sharing the same key.

Validation dict.