In you code snippet you are accumulating the model output feature_ext in feature_sum_mid, which will also store the computation graph (including all intermediate tensors).
If you want to use this Center tensor as a constant target, you should wrap the calculation in compute_Center into with torch.no_grad() to avoid storing the computation graphs and thus lower the memory usage.
Also, Variables are deprecated since PyTorch 0.4, so you can use tensors in newer versions.