Serialize optimized onnx model (#1470)
* Model serialization
* Removed duplicate symbol
* Minor update
* Review comments
* add tests
* Model serialization
* Removed duplicate symbol
* Minor update
* Merged PR 1106437: Model Serialization in onnxruntime
* Review comments
* Merged PR 1107226: Review comments
Review comments
* add tests
* Fixed merge conflict
* Correct python tests
* InferenceSesssion Refeed Test
* Replace use of widechar const literal-L
* Fixed failing tests
* Updated comment
* Removed unnecessary session options
* Spell check on comments
* Do not serialize when level 3 optimization specified
* Updated error logs
* Changed log severity to WARN
