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**Description**
Package frequently-used subprocess invoke into function.

**Description**
Replace `sum(results) / len(results)` with `statistics.mean(results)`

**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.

* Unify SB_MICRO_PATH and SB_MICRO_LIB

* fix bug of lib path

* Revise MicroBenchmark class to be more flexible.
* use command index not the command as the parameter.
* changes according to discussion.

* revise result process interface

* add more comments
Co-authored-by: Guoshuai Zhao <guzhao@microsoft.com>

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