create different trainers for different tasks

[satyajitghana]

Now that we have to train GANs as well, why not have different Trainers for different tasks, like

ClassifierTrainer
GANTrainer
LSTMTrainer
BERTTrainer

each of the trainer is responsible to know how to train that kind of network and has the training loop and test loop in them, we can expose an api for users to add their own Trainers !