What do you mean?
If you want to get the current running_mean and running_var in a pretrained model, use model.{your_batch_norm_module}.running_mean and model.{your_batch_norm_module}.running_var.
If you want to get the running_mean and running_var in a pretrained model after forward x, use torch.nn.functional.batch_norm or just forward that layer.
If you donot have a pretrained model, and want to get the running_mean and running_var, init running_mean to 0 and running_var to 1 then use torch.nn.functional.batch_norm or torch.batch_norm.