Support onnx export with previous OpSet version

[gh-yewang]

For Feature request: #5171

Support exporting to onnx with a lower opset version
upgrade opset version from 11 to 12

[codecov]

Codecov Report

Merging #5176 into master will decrease coverage by 0.00%.
The diff coverage is 74.35%.

@@            Coverage Diff             @@
##           master    #5176      +/-   ##
==========================================
- Coverage   73.32%   73.31%   -0.01%     
==========================================
  Files        1007     1007              
  Lines      187911   188075     +164     
  Branches    20241    20244       +3     
==========================================
+ Hits       137790   137895     +105     
- Misses      44595    44657      +62     
+ Partials     5526     5523       -3     

Flag	Coverage Δ
#Debug	73.31% <74.35%> (-0.01%)	⬇️
#production	69.08% <98.24%> (+<0.01%)	⬆️
#test	87.40% <51.66%> (-0.05%)	⬇️

Impacted Files	Coverage Δ
src/Microsoft.ML.Data/Model/Onnx/OnnxContext.cs	100.00% <ø> (ø)
test/Microsoft.ML.Tests/OnnxConversionTest.cs	96.81% <51.66%> (-2.16%)	⬇️
src/Microsoft.ML.OnnxConverter/OnnxContextImpl.cs	72.91% <88.88%> (+0.38%)	⬆️
src/Microsoft.ML.Data/Prediction/Calibrator.cs	80.28% <100.00%> (+0.01%)	⬆️
....Data/Transforms/ColumnConcatenatingTransformer.cs	84.64% <100.00%> (+0.02%)	⬆️
src/Microsoft.ML.Data/Transforms/ColumnCopying.cs	97.19% <100.00%> (+0.02%)	⬆️
...rc/Microsoft.ML.Data/Transforms/ColumnSelecting.cs	96.29% <100.00%> (+0.01%)	⬆️
src/Microsoft.ML.Data/Transforms/Hashing.cs	81.41% <100.00%> (+0.01%)	⬆️
src/Microsoft.ML.Data/Transforms/KeyToValue.cs	71.57% <100.00%> (+0.07%)	⬆️
src/Microsoft.ML.Data/Transforms/KeyToVector.cs	83.67% <100.00%> (+0.03%)	⬆️
... and 57 more

[harishsk]