Deep Reinforcement Learning Methods - Week 9

This week I released the PR for async one step q learning and async one step sarsa, it's under review now and I believe it will be merged soon. I also worked on A3C. I implemented a wrapper network for actor and critic, and added a new reinforce layer for policy gradient. Current architecture of ANN doesn't support shared layers, which is necessary in A3C. Use shared_ptr can address this problem, however it may lead to overhead and may make it inconvenient for user to add new layer types. After discussing with Ryan, we decided to use AliasLayer, the main challenge is to make it compatible with member function checkers like HasParameters(). I thought I could address this issue by overloading or specializing boost::apply_visitor before, but soon I realized it's impossible. Maybe I have to add overload functions for AliasLayer template argument for all visitors.