dlg in attack module: attack fedavg, the loss is still high after searching the dummy data

[Osier-Yi]

Describe the bug
When running the provided dlg attack script, the loss of dlg after the max iteration rounds is still very high, and cannot recover the original training data.

To Reproduce
Run the provided dlg attack script.

Expected behavior
The loss should be close to 0 after several iterations and the original training data can be recovered.