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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fit_transform(self, X, y=None, **fit_params):
 """Fit all transformers, transform the data and concatenate results. Parameters X : iterable or array-like, depending on transformers Input data to be transformed. Targets for supervised learning. Returns ------- X_t : array-like or sparse matrix, shape (n_samples, sum_n_components) hstack of results of transformers. sum_n_components is the sum of n_components (output dimension) over transformers. """

        self._validate_transformers()
        with Pool(self.n_jobs) as pool:
            result = pool.starmap(_fit_transform_one,
                         ((trans, weight, X[trans['col_pick']] if hasattr(trans, 'col_pick') else X, y)
                          for name, trans, weight in self._iter()))
        if not result:
            # All transformers are None
            return np.zeros((X.shape[0], 0))
        Xs, transformers = zip(*result)
        self._update_transformer_list(transformers)
        if self.concatenate:
            if any(sparse.issparse(f) for f in Xs):
                Xs = sparse.hstack(Xs).tocsr()
            else:
                Xs = np.hstack(Xs)
        return Xs