[megatron_gpt2] checkpoint v3
The BigScience generates Megatron-Deepspeed checkpoints en masse, so we need a better v3 checkpoint support for megatron_gpt2 (also working on a direct Megatron-Deepspeed to HF-style script).

This PR:
- removes the requirement that the source checkpoint is a zip file. Most of the checkpoints aren't .zip files, so supporting both.
- removes the need to manually feed the config, when all the needed data is already in the checkpoint. (while supporting the old format)
- disables a debug print that repeats for each layer
- switch to default `gelu` from `gelu_new` - which is what the current megatron-lm uses
- fixes a bug in the previous version. The hidden size is dimension 1 as it can be seen from:
https://github.com/NVIDIA/Megatron-LM/blob/3860e995269df61d234ed910d4756e104e1ab844/megatron/model/language_model.py#L140-L141
The previous script happened to work because `max_position_embeddings` happened to be equal to `hidden_size`

@LysandreJik, @sgugger 
