* Load/save the params from the params_path
* Switch to use load_persistables and save_persistables
* Instaed of setup the executor to run program and scope. Pass the program to the load_persistables
- The trainer and inferencer will load params from disk if param_path
argument is not None in their constructor.
- Remove params.py, we will expose core.Scope to the user if needed
(e.g., for GAN). Currently we will not expose it, unless we clearly
know doing so can support GAN.
- Add `save_params` to Trainer (a TODO item).
- rename "network" to "program"