Add system metadata class

[odashi]

Processor.process() takes metadata, which is used to directly initialize SysOutputInfo. However, these are essentially different data (especially, "metadata" $\subset$ SysOutputInfo, but not $=$) and the current implementation makes some confusion around this:

The most significant abuse around this behavior is that FileLoaderMetadata is implicitly converted into SysOutputInfo. This shouldn't work unless explicit conversion:

ExplainaBoard/integration_tests/ner_test.py

Lines 148 to 154 in 4cec0a0

	metadata = dataclasses.asdict(data.metadata)
	metadata.update(
	{
	"task_name": TaskType.named_entity_recognition.value,
	}
	)
	processor = get_processor_class(TaskType.named_entity_recognition)()

To this end, we need:

• A struct defining the system metadata.
• Change the behavior of Processor to take the system metadata, not a dict.
• Either:
  • A conversion method between system metadata and FileLoaderReturn/SysOutputInfo
  • Include system metadata as a direct member of FileLoaderReturn/SysOutputInfo

[odashi]

RFC: @neubig @pfliu-nlp

[neubig]

Sounds like a good idea, I agree

[pfliu-nlp]

"The most significant abuse around this behavior is that FileLoaderMetadata is implicitly converted into SysOutputInfo."

Agree. Adding system metadata class sounds nice.