import torch


def quaternion_to_axis_angle(quaternions):
    """
    Convert rotations given as quaternions to axis/angle.
    Args:
        quaternions: quaternions with real part first,
            as tensor of shape (..., 4).
    Returns:
        Rotations given as a vector in axis angle form, as a tensor
            of shape (..., 3), where the magnitude is the angle
            turned anticlockwise in radians around the vector's
            direction.
    """
    norms = torch.norm(quaternions[..., 1:], p=2, dim=-1, keepdim=True)
    half_angles = torch.atan2(norms, quaternions[..., :1])
    angles = 2 * half_angles
    eps = 1e-6
    small_angles = angles.abs() < eps
    sin_half_angles_over_angles = torch.empty_like(angles)
    sin_half_angles_over_angles[~small_angles] = (
            torch.sin(half_angles[~small_angles]) / angles[~small_angles]
    )
    # for x small, sin(x/2) is about x/2 - (x/2)^3/6
    # so sin(x/2)/x is about 1/2 - (x*x)/48
    sin_half_angles_over_angles[small_angles] = (
            0.5 - (angles[small_angles] * angles[small_angles]) / 48
    )
    return quaternions[..., 1:] / sin_half_angles_over_angles