Gpt-2-simple

* Support the 345M model (thanks to Neil Shepperd for the [gradient checkpointing](https://github.com/nshepperd/gpt-2/commit/47df6da611716b4826e3397cd68d711c6951c8e5) implementation!)
* Support model_name in the CLI for above support
* Support run_name in the CLI
* Support `.csv` files as an input dataset to `finetune` (will parse the CSV as if it was done via `encode_csv()`).
* Fix one off issues (21)

* Fix one-off error where checkpoint saved a step early.
* Fix issue where `restore_from='fresh` uses the counter from a previously-trained checkpoint.
* If `restore_from='latest` , `steps` will now train for the specified amount of steps, instead of the training until the specified number of steps. (13, 14)

* Added a basic CLI.
* Added a `include_prefix` parameter to give an option to exclude the input prefix.
* Improved regex for truncation.

* `is_gpt2_downloaded`: Check if the model is downloaded.
* `encode_csv`: Convert a CSV to a format suitable for GPT-2.