The l2 norm of an array is is defined as: sqrt(||x||), where ||x|| is the
    dot product of the vector.

def l2_norm(arr):
    """
    The l2 norm of an array is is defined as: sqrt(||x||), where ||x|| is the
    dot product of the vector.
    """
    arr = np.asarray(arr)
    return np.sqrt(np.dot(arr.ravel().squeeze(), arr.ravel().squeeze()))