Backprop minus real error for DP-GAN

[JimAchterbergLUMC]

Description

The discriminator loss in a GAN consists of -err_real+err_fake.
When differential privacy is enabled, we can separately backpropagate the fake error (with hooks disabled) and real error (with hooks enabled).

However, the current code backpropagates err_fake and err_real, but I believe this should be err_fake and -err_real, to match the non-differentially private loss?

i.e., we should backpropagate "minus the real error" instead of simply the "real error", when differential privacy is enabled for GANs.

How to Reproduce

1. Go to 'synthcity/plugins/core/models/gan.py', line 452. Here we see that the positive real error is backpropagated. However, looking at line 438, I believe this should be -(err_real).

Expected Behavior

Change line 452 in 'synthcity/plugins/core/models/gan.py' from errD_real.backward(), to (-errD_real).backward().