[megatron_gpt2] dynamic gelu, add tokenizer, save config
This PR improves `megatron_gpt2/convert_megatron_gpt2_checkpoint.py`:
- dynamically figures out the correct activation function based on how megatron was trained
- dynamically figures out the correct tokenizer and sets `config.tokenizer_class`
- adds tokenizer files
- switches to `config.save_pretrained` to ensure the config gets the right bits of the most recent `transformers`

Some of this functionality has been discussed here https://github.com/huggingface/transformers/issues/13906