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: https://github.com/neulab/ExplainaBoard/blob/4cec0a01cbe2617e9a67a440be25ee4252f792b2/integration_tests/ner_test.py#L148-L154

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.