[-0.2951, -1.3350, -0.7694,  0.5600],
              [ 1.0842, -0.9580,  0.3623,  0.2343]])
      >>> torch.mean(a, 1)
      tensor([-0.0163, -0.5085, -0.4599,  0.1807])
      >>> torch.mean(a, 1, True)
      tensor([[-0.0163],
              [-0.5085],
              [-0.4599],
              [ 0.1807]])