For example
>>> x
tensor([[[[ 1.,  3.],
          [ 4.,  6.]],

         [[-1., -3.],
          [-4., -7.]]]])
>>> x.mean([3, 0, 1])
tensor([ 0.0000, -0.2500])
>>> x.mean([3, 0])
tensor([[ 2.0000,  5.0000],
        [-2.0000, -5.5000]])
>>> x.mean([3, 0]).mean([1])
tensor([ 3.5000, -3.7500])

I had believed that, doing mean([3, 0, 1]) is equivalent of doing x.mean([3, 0]).mean([1])