Initialize the iterator given input.

def _init_iter(self, X, y, is_train):
        """Initialize the iterator given input."""
        if isinstance(X, (np.ndarray, nd.NDArray)):
            if y is None:
                if is_train:
                    raise ValueError('y must be specified when X is numpy.ndarray')
                else:
                    y = np.zeros(X.shape[0])
            if not isinstance(y, (np.ndarray, nd.NDArray)):
                raise TypeError('y must be ndarray when X is numpy.ndarray')
            if X.shape[0] != y.shape[0]:
                raise ValueError("The numbers of data points and labels not equal")
            if y.ndim == 2 and y.shape[1] == 1:
                y = y.flatten()
            if y.ndim != 1:
                raise ValueError("Label must be 1D or 2D (with 2nd dimension being 1)")
            if is_train:
                return io.NDArrayIter(X, y, min(X.shape[0], self.numpy_batch_size),
                                      shuffle=is_train, last_batch_handle='roll_over')
            else:
                return io.NDArrayIter(X, y, min(X.shape[0], self.numpy_batch_size), shuffle=False)
        if not isinstance(X, io.DataIter):
            raise TypeError('X must be DataIter, NDArray or numpy.ndarray')
        return X