Hey @jackieliu930,
When using run_summarization.py with a T5 like the model you need to add an additional hyperparameter source_prefix: "summarize: ".

Only T5 models t5-small , t5-base , t5-large , t5-3b and t5-11b must use an additional argument: --source_prefix "summarize: " .

You can find more information about the run_summarization.py here:transformers/examples/pytorch/summarization at master · huggingface/transformers · GitHub