text: Title: Add data loader for HF oasst1
  Body: Currently, data needs to be manually downloaded and a path specified via config to train on OpenAssistant datasets (both internal and https://huggingface.co/datasets/OpenAssistant/oasst1/blob/main/2023-04-12_oasst_ready.trees.jsonl.gz) so that the tree can be parsed by [`model_training.custom_datasets.oasst_dataset.load_oasst_export`](https://github.com/LAION-AI/Open-Assistant/blob/63937be7462ddc3a46399e9a813b36c2585a6fb3/model/model_training/custom_datasets/oasst_dataset.py#L21)
  
  We should write/refactor the data loader to work directly with the dataset returned by HF `datasets.load_dataset("OpenAssistant/oasst1")`