Reduce the number of required arguments

[neubig]

From @pfliu-nlp in this comment: As a demonstration example in README.md, I think this would make the product more attractive: "shortest code makes most powerful things". Based on this excerpt code could be further simplified in the future, for example,

• We can reduce the need for task name, source_language, which could be obtained in named datasets
• We can even remove the system_name, split, system_output_file_type, by providing some default values.

Of course, we can instruct users to see more comprehensive usage of general examples.