Add the ability to run the EleutherAI Evaluation Harness on Megatron checkpoints. Right now we are relying on converting Megatron checkpoints to Hugginface checkpoints which is an error-prone process. We also have to use Megatron anyways to run the 200B model.

Implementation details:
You will use this HF gpt2 model implementation here as your reference. Here are more details:

• Edit _init_, create_from_arg_string to load the Megatron checkpoints
• Edit the _model_call function to call the Megatron model and read logits back
• The functions loglikelihood, loglikelihoods, _loglikelihood_tokens might (or might not) require a little tweaking
• Leave the function greedy_until unimplemented (raise an exception), we don't need it for now.
• Check this test that shows how to load and call a Megatron checkpoint.
• Here's one Megatron checkpoint that you can work with.
• A relatively close implementation is already in the GPT-NeoX repo here and it might be helpful to check as well.