Hi,

I find there is slight difference between the calculation results of nn.BatchNorm2d.forward() and torch.Tensor. Specifically, If I run the following code:

```
import torch
import torch.nn as nn
bn = nn.BatchNorm2d(10)
# feed data to update running_mean and running_var
for _ in range(100):
dummy_data = torch.randn(8,10,100,100)
_ = bn(dummy_data)
# now test
bn.eval()
#result of layer forward
data_in = torch.randn(8,10,100,100)
output_bn = bn(data_in)
#result of manual calculation
mean = bn.running_mean.view(1, 10, 1, 1)
var = bn.running_var.view(1, 10, 1, 1)
weight = bn.weight.view(1, 10, 1, 1)
bias = bn.bias.view(1, 10, 1, 1)
eps = bn.eps
data_in_norm = (data_in - mean) / torch.sqrt(var+eps)
output_mannual_calcualte = weight*data_in_norm + bias
#calculate error
err = (output_bn - output_mannual_calcualte).abs()
print(torch.max(err))
```

It will print something like:

```
tensor(4.7684e-07, grad_fn=<MaxBackward1>)
```

Basically, it means the results are slightly different between the forward function of nn.BatchNorm2d and manually doing the same calculation using torch.Tensor. I wonder if there is something wrong in my manual calculation? Am I missing something here? Thank you very much for any suggestions.