Gpt-2-simple

Thanks to https://github.com/YaleDHLab via https://github.com/minimaxir/gpt-2-simple/pull/275, gpt-2-simple now supports TensorFlow 2 by default, and the minimum TensorFlow version is now 2.5.1! The Colab Notebook has also been update to no longer use TensorFlow 1.X.

Note: Development on gpt-2-simple has mostly been superceded by [aitextgen](https://github.com/minimaxir/aitextgen), which has similar AI text generation capabilities with more efficient training time and resource usage. If you do not require using TensorFlow, I recommend using aitextgen instead. Checkpoints trained using gpt-2-simple can be [loaded using aitextgen](https://docs.aitextgen.io/gpt-2-simple/) as well.

* Switched the model URL from GCP to Azure. (253)
* Pin TensorFlow 1.15 (200)
* Add checkpoint loading from other checkpoints (175)

Some have successfully finetuned 774M/1558M, so the assert has been removed.

* Multi-GPU support (127) (not fully tested; will add some docs when done)
* Fixed checkpoint dir bug (134)
* Added a hard assert of a TensorFlow version >= 2.0 is used (137)

* 774M is explicitly blocked from being fine-tuned and will trigger an assert if attempted. If a way to finetune it without being super-painful is added, the ability to finetune it will be restored.
* Allow ability to generate text from the default pretrained models by passing `model_name` to `gpt2.load_gpt2()` and `gpt2.generate()` (this _will_ work with 774M.
* Add`sgd` as an `optimizer` parameter to `finetune` (default: `adam`)
* Support for changed model names, w/ changes more prominent in the README.

Merged a few PRs:

Fixed generate cmd run name: 78
Resolved most depreciation warnings: 83
Optional model parameters: 90

This does not make the package fully TF 2.0 compatible, but it's a big step!