bugfix: saving DeepSAD model with pretrain=False

[youph]

Before the fix, saving the model failed when pretrain was set to False, with the following error:

  File "/Deep-SAD-PyTorch/src/DeepSAD.py", line 133, in save_model
    ae_net_dict = self.ae_net.state_dict() if save_ae else None
AttributeError: 'NoneType' object has no attribute 'state_dict'

This fix checks that self.ae_net is not None before saving the autoencoder.