Clamp the weights with `self.clip` if it's not None, return the correct input.

def on_batch_begin(self, last_input, last_target, **kwargs):
        "Clamp the weights with `self.clip` if it's not None, return the correct input."
        if self.clip is not None:
            for p in self.critic.parameters(): p.data.clamp_(-self.clip, self.clip)
        return {'last_input':last_input,'last_target':last_target} if self.gen_mode else {'last_input':last_target,'last_target':last_input}