[megatron convert] PYTHONPATH requirements
This PR documents how to tell the megatron conversion scripts to find the `Megatron-LM` repo, which is needed for recent checkpoints as it was reported at https://github.com/huggingface/transformers/issues/14939

Since one can't install Megatron-LM as a package we can't make the script require it, so documenting it here and will also document in the model cards.

Fixes: https://github.com/huggingface/transformers/issues/14939

@LysandreJik 

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@jdemouth already updated these:

- https://huggingface.co/nvidia/megatron-gpt2-345m/blob/main/README.md
- https://huggingface.co/nvidia/megatron-bert-uncased-345m/blob/main/README.md
- https://huggingface.co/nvidia/megatron-bert-cased-345m/blob/main/README.md

I need to figure out how to get perms to do so. 