Save ConvertTransform as ONNX Operator and Control the Use of Experimental Features with a Flag

[wschin]

1. Introduce a new argument to SaveOnnx, which is OnnxVersion.
   Two values are currently allowed, "Latest" and "Experimental".
   Note that "Latest" means that the produced ONNX model meets
   the latest ONNX release while "Experimental" may produce
   things not officially supported in ONNX.
2. For (1), the interface of saving ONNX is slightly changed. Now,
   CanSaveOnnx requires an OnnxContext as its input argument,
   because if a model can be saved to ONNX depends on the targeted
   ONNX version now.
3. Add exporter for ConvertTransform. It doesn't use standard
   ONNX operator.

Related issue: #945

[dnfclas]

All CLA requirements met. #Resolved

[markusweimer]

This PR does not reference an issue. Can you please file one and reference it in the PR description? #Resolved

[wschin]

@markusweimer , sure. Thanks for reminding. #Resolved

[Ivanidzo4ka]

Can you make sure it pass build system? #Resolved

[codemzs]

[Ivanidzo4ka]

[justinormont]

Looks like there's a test which failed:

2018-10-09T17:45:52.5177105Z Failed   Microsoft.ML.Runtime.RunTests.TestEntryPoints.EntryPointCatalog
2018-10-09T17:45:52.5177853Z Error Message:
2018-10-09T17:45:52.5178689Z  Assert.True() Failure
2018-10-09T17:45:52.5178806Z Expected: True
2018-10-09T17:45:52.5179047Z Actual:   False
2018-10-09T17:45:52.5179125Z Stack Trace:
2018-10-09T17:45:52.5179196Z    at Microsoft.ML.Runtime.RunTests.BaseTestBaseline.Done() in /__w/1/s/test/Microsoft.ML.TestFramework/BaseTestBaseline.cs:line 149
2018-10-09T17:45:52.5179510Z    at Microsoft.ML.Runtime.RunTests.TestEntryPoints.EntryPointCatalog() in /__w/1/s/test/Microsoft.ML.Core.Tests/UnitTests/TestEntryPoints.cs:line 300
2018-10-09T17:45:52.5180265Z Standard Output Messages:
2018-10-09T17:45:52.5181202Z  Output matches baseline: '../Common/EntryPoints/core_ep-list.tsv'
2018-10-09T17:45:52.5181605Z  *** Failure: Output and baseline mismatch at line 2492: '../Common/EntryPoints/core_manifest.json'
2018-10-09T17:45:52.5181707Z  Test EntryPointCatalog: completed normally: failed
2018-10-09T17:45:52.5181824Z 
2018-10-09T17:45:52.5182157Z 
2018-10-09T17:45:52.5709539Z Results File: /__w/1/s/bin/AnyCPU.Debug/Microsoft.ML.Core.Tests/_d036fad37c59_2018-10-09_17_44_56.trx
2018-10-09T17:45:52.5712294Z 
2018-10-09T17:45:52.5715346Z Total tests: 105. Passed: 101. Failed: 1. Skipped: 3.
2018-10-09T17:45:52.5717716Z Test Run Failed.
2018-10-09T17:45:52.5734548Z Test execution time: 59.2323 Seconds