You could use something like:
small_bn = nn.BatchNorm2d(1)
with torch.no_grad():
    small_bn.weight = nn.Parameter(large_bn.weight[0:1]) # or take the mean, slice at another pos?
    # same for bias, running_mean and running_var

I don’t know, if that would make sense at all for your use case, but you might consider it.