'Returns the (flattened, log-transformed) non-fixed hyperparameters.
Note that theta are typically the log-transformed values of the
kernel\'s hyperparameters as this representation of the search space
is more amenable for hyperparameter search, as hyperparameters like
length-scales naturally live on a log-scale.
Returns
theta : array, shape (n_dims,)
The non-fixed, log-transformed hyperparameters of the kernel'

@property
def theta(self):

    return np.hstack([kernel.theta for kernel in self.kernels])
