Provide the shuffled index_mapping npy files for ease of reproducing training data

Hi,

I was wondering can you provide the index_mapping files that is generated by the [GPT2Dataset](https://github.com/EleutherAI/gpt-neox/blob/03186decef022dc35e6adee1a66619968812e0a9/megatron/data/gpt2_dataset.py#L29)? From the construction of gpt2dataset at [here](https://github.com/EleutherAI/gpt-neox/blob/03186decef022dc35e6adee1a66619968812e0a9/megatron/data/gpt2_dataset.py#L158), I can see there are three `npy` index files
```
doc_idx_filename = _filename + "_doc_idx.npy"
sample_idx_filename = _filename + "_sample_idx.npy"
shuffle_idx_filename = _filename + "_shuffle_idx.npy"
```
I was wondering can you provide a copy of these files so that I don't need to regenerate them?

I ask this request because I want to study the influence of the original training data by chunk. I have prepared the pythia-dedup dataset, but I failed to build the environments. After reading the code of [GPT2Dataset](https://github.com/EleutherAI/gpt-neox/blob/03186decef022dc35e6adee1a66619968812e0a9/megatron/data/gpt2_dataset.py#L29), I found that with these index files, I can reproduce the original training data of pythia.

I noticed that you provide the `batch_viewer.py` to check the unshuffled data, but it seems that these data is still different from the actually training data that is fed into the model during the training process.

Thanks