onnxruntime

update C# API to optimize inference latency (#3171)

* update C# API to optimize inference latency

* rename PinnedOnnxValue to fixedBufferOnnxValue and fix build break

* add more test cases

* add conditions on string tensors for pre-allocated outputs

* change to random inputs

* fix word spell

* resolve comments

* resolve comments

* remove FixedBufferOnnxValueTests.cs

* fix trivial typos in doc