Fit InvertableHashingVectorizer on doc inside a FeatureUnion.

def _fit_invhashing_union(vec_union, docs):
    # type: (FeatureUnion, Any) -> FeatureUnion
    """ Fit InvertableHashingVectorizer on doc inside a FeatureUnion.
    """
    return FeatureUnion(
        [(name, invert_hashing_and_fit(v, docs))
         for name, v in vec_union.transformer_list],
        transformer_weights=vec_union.transformer_weights,
        n_jobs=vec_union.n_jobs)