transformers pytorch rust onnx safetensors longt5 text2text-generation summarization summary booksum long-document long-form dataset:kmfoda/booksum arxiv:2112.07916 arxiv:2105.08209 doi:10.57967/hf/2078 license:apache-2.0 license:bsd-3-clause model-index autotrain_compatible endpoints_compatible region:us