running_mean = (1 - momentum) * running_mean + momentum * x_mean

https://pytorch.org/docs/stable/generated/torch.nn.BatchNorm2d.html?highlight=batchnorm#torch.nn.BatchNorm2d 1
.. note::
        This :attr:`momentum` argument is different from one used in optimizer
        classes and the conventional notion of momentum. Mathematically, the
        update rule for running statistics here is
        :math:`\hat{x}_\text{new} = (1 - \text{momentum}) \times \hat{x} + \text{momentum} \times x_t`,
        where :math:`\hat{x}` is the estimated statistic and :math:`x_t` is the
        new observed value.