running_mean is tensor rather than Variable, which means you should not set attr data for tensor, it makes no sense.
import torch
import torch.nn as nn

bn = nn.BatchNorm2d(4)
bn.weight.data  = torch.arange(0,4)
bn.bias.data  = torch.arange(4,8)
bn.running_mean = torch.arange(8,12)
bn.running_var = torch.arange(12,16)
inp = torch.autograd.Variable(torch.randn(1,4,4,4))
out = bn(inp)
print bn.running_mean