API Change

1. add a new class DataParallelNet for multi-GPU training (#461)
2. add ActorCritic for deterministic parameter grouping for share-head actor-critic network (#458)
3. collector.collect() now returns 4 extra keys: rew/rew_std/len/len_std (previously this work is done in logger) (#459)
4. rename WandBLogger -> WandbLogger (#441)

Bug Fix

1. fix logging in atari examples (#444)

Enhancement

1. save_fn() will be called at the beginning of trainer (#459)
2. create a new page for logger (#463)
3. add save_data and restore_data in wandb, allow more input arguments for wandb init, and integrate wandb into test/modelbase/test_psrl.py and examples/atari/atari_dqn.py (#441)