Add "offline" data cache generation support
What is the reasoning behind overloading num_workers for the num_dataset_builder_threads, instead of passing num_dataset_builder_threads in as a separate config parameter? 

I could imagine a case where I set num_dataset_builder_threads to a very large number (64 or 128) but wouldn't want that many workers during the typical data loading. That would allow many workloads to specify a high num_dataset_builder_threads without needing to explicitly use the data_cache_generation_only offline step.

Instead I would suggest num_dataset_builder_threads as an explicit new parameter in megatron_gpt_config.yaml, and default it to 1 to mirror https://github.com/NVIDIA/Megatron-LM/blob/e33c8f78a35765d5aa37475a144da60e8a2349d1/megatron/core/datasets/blended_megatron_dataset_config.py#L48 > What is the reasoning behind overloading num_workers for the num_dataset_builder_threads, instead of passing num_dataset_builder_threads in as a separate config parameter?
> 
> I could imagine a case where I set num_dataset_builder_threads to a very large number (64 or 128) but wouldn't want that many workers during the typical data loading. That would allow many workloads to specify a high num_dataset_builder_threads without needing to explicitly use the data_cache_generation_only offline step.
> 
> Instead I would suggest num_dataset_builder_threads as an explicit new parameter in megatron_gpt_config.yaml, and default it to 1 to mirror https://github.com/NVIDIA/Megatron-LM/blob/e33c8f78a35765d5aa37475a144da60e8a2349d1/megatron/core/datasets/blended_megatron_dataset_config.py#L48

thank you for the review, fixed.
# What does this PR do ?

1. Adds "offline" data cache generation support. The training script will be stopped once data cache is generated.
2. Enable mcore's `num_dataset_builder_threads` param usage.



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