[Refactoring] Shared configuration within frontend parser, graph/ops lower and backend

[dboyliao]

Shared configuration such as padding and activation.
ex:
TF 1.x pb file: padding is raw string such as 'VALID', 'SAME', ...etc
TF 2.x tflite file: padding is int index such as '0' ('UNKNOWN'), '1' ('VALID'), ...etc

Need a better way to share such configurations