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**Description**
Set the `reduce_op` type for metirc `return_code` as `None`.

**Description**
Add return_code metric into result and revise unit tests.

**Description**
Change the field name `reduce` to `reduce_op`.

**Description**
Add reduce function support for output summary.

**Major Revision**
- Add reducer class to maintain all reduce functions.
- Save reduce type of each metric into `BenchmarkResult`
- Fix UT.

* benchmarks init.
Co-authored-by: Guoshuai Zhao <guzhao@microsoft.com>