Add in checkpointing capability

[muellerzr]

Closes #171

This PR adds in two functions, Accelerator.save_state and Accelerator.load_state, which will go through and checkpoint the current state of everything accelerator touches.

I've included a testing script, but this needs to be properly turned into a test, as currently it is just a script with a bunch of asserts. I have tested successfully on:

• Single GPU
• Single CPU
• Multi-GPU

[sgugger]

Thanks a lot for working on this! I've left a few comments on the general API and some nits. The most important thing is to figure out how to save the LR scheduler with the rest :-)

[sgugger]

Thanks a lot for continuing your work on this. Just left some nits and one last comment on the test you added, then we can merge this!