Add tests for Adversary at arbitrary layer in model.

[dxoigmn]

Right now, the Adversary checks whether model is present in order to determine when to attack: https://github.com/IntelLabs/MART/blob/a2f936e5c4486e179fd8e47d03301b0f8bd16e9a/mart/attack/adversary.py#L314

However, because an Adversary can live at any layer, it should really check whether sequence is present: https://github.com/IntelLabs/MART/blob/a2f936e5c4486e179fd8e47d03301b0f8bd16e9a/mart/nn/nn.py#L115

The reason is that the sequence represents the "continuation" of the model at the point where the Adversary lives in the model. Meaning that the Adversary can call the sequence to get a loss. Calling the model may be incorrect depending upon at which layer of model the Adversary lives.