Verifies eval shared models and normnalizes to produce a single list.

The output is normalized such that if a list or dict contains a single entry, the model name will always be empty.

eval_shared_model None, a single model, a list of models, or a dict of models keyed by model name.

A list of models or None.

ValueError if dict is passed and keys don't match model names or a multi-item list is passed without model names.