Allow pickle for saving checkpoints in Experiment

[nmatthews-asapp]

Sometimes our custom save format doesn't work well for certain architectures, or users would just prefer pickle, or because the save format isn't as mature as pickle there can be bugs. For all these reasons, we should enable pickle checkpointing in Experiment

[nmatthews-asapp]

Note: pickle (via torch.save) will be the standard in v0.5; it is very difficult to implement in 0.4 so it will not be implemented there