<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fit(self, dataset):
 """ Computes the inverse document frequency. :param dataset: an RDD of term frequency vectors """

        if not isinstance(dataset, RDD):
            raise TypeError("dataset should be an RDD of term frequency vectors")
        jmodel = callMLlibFunc("fitIDF", self.minDocFreq, dataset.map(_convert_to_vector))
        return IDFModel(jmodel)