Add a binding for the torch.nn.elu activation function

[Filippo Poltronieri]

I am using torch.rb to implement a reinforcement learning library in Ruby reinforce.
I was wondering if you are planning to add the Torch::NN::Elu activation function in torch.rb. That would help me a lot in writing a DeepSet PPO implementation.

Thank you very much!

Best Regards

[Andrew Kane]

Hi @fpoltronieri, added in the commit above. Let me know if you come across any other missing functionality that you need.