rgument of the returned kernel k(X, Y)

Y : array-like of shape (n_samples_X, n_features) or list of object,                default=None
    Right argument of the returned kernel k(X, Y). If None, k(X, X)
    is evaluated instead.

eval_gradient : bool, default=False
    Determines whether the gradient with respect to the log of
    the kernel hyperparameter is computed.

Returns
-------
K : ndarray of shape (n_samples_X, n_samples_Y)
    Kernel k(X, Y)

K_gradient : ndarray of shape (n_samples_X, n_samples_X, n_dims),                optional
    The gradient of the kernel k(X, X) with respect to the log of the
    hyperparameter of the kernel. Only returned when `eval_gradient`
    is True.
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