[Model] Add HuggingFace GPT-2

[comaniac]

Description

Add an example of HuggingFace GPT-2. Later we should rename examples/gpt to examples/gpt-neo to reduce the confusion.

Example benchmark commands:

deepspeed ./examples/gpt2/deepspeed_hf.py --pmp 2 --tmp 2 --batch_size 32 --micro_batch_size 4 --model_name gpt2-xl --iter_nums 170 --hidden-size 2048 --nlayers 24 --num-attn-heads 16 --dropout 0.1 --activation_function gelu --seq_len 2048

Checklist

• PR's title starts with a category (e.g. [Bugfix], [Model], [Tutorial], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

cc @szhengac