Implement other native attacks

[maurapintor]

Other attacks can be implemented with the native backend. Possibly, the implementation should allow custom choice of loss, optimizer, and other customizable components.

Open an issue and a separate branch for each attack.

• FMN https://arxiv.org/abs/2102.12827
• CW https://arxiv.org/abs/1608.04644
• DDN https://arxiv.org/abs/1811.09600
• ... others

References:

• the base attack class https://github.com/pralab/secml-torch/blob/main/src/secmlt/adv/evasion/base_evasion_attack.py
• the PGD implementation https://github.com/pralab/secml-torch/blob/main/src/secmlt/adv/evasion/modular_attack.py