Implement lite custom op API (#15778)
Implement a set of new APIs for lightweight custom ops registration, to
save efforts from schema-composing.
A few highlights:
- Support build-time type inference;
- Support function-as-op for "stateless" ops;
- Support structure-as-op for "stateful" ops;
- Support varied input/output forms such as span, scalar, and tensors,
either optional or non-optional.
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Co-authored-by: Randy Shuai <rashuai@microsoft.com>
