Rename Save API for DataLoader to SaveDataLoader.

[codemzs]

towards #2991

[codecov]

Codecov Report

Merging #3018 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #3018     +/-   ##
=========================================
- Coverage   72.33%   72.14%   -0.2%     
=========================================
  Files         803      803             
  Lines      143424   143422      -2     
  Branches    16169    16169             
=========================================
- Hits       103752   103465    -287     
- Misses      35250    35545    +295     
+ Partials     4422     4412     -10

Flag	Coverage Δ
#Debug	72.14% <100%> (-0.2%)	⬇️
#production	68.06% <100%> (+0.01%)	⬆️
#test	87.53% <100%> (-1%)	⬇️

Impacted Files	Coverage Δ
.../Microsoft.ML.Data/Model/ModelOperationsCatalog.cs	97.54% <100%> (+0.26%)	⬆️
test/Microsoft.ML.Functional.Tests/ModelLoading.cs	93.26% <100%> (-0.43%)	⬇️
test/Microsoft.ML.TestFramework/RemoteExecutor.cs	0% <0%> (-75.31%)	⬇️
test/Microsoft.ML.TestFramework/PasteArguments.cs	0% <0%> (-42.86%)	⬇️
...osoft.ML.CpuMath.UnitTests.netcoreapp/UnitTests.cs	69.83% <0%> (-29.57%)	⬇️
...c/Microsoft.ML.CpuMath/CpuMathUtils.netstandard.cs	89.93% <0%> (-3.82%)	⬇️
src/Microsoft.ML.Maml/MAML.cs	26.21% <0%> (+1.45%)	⬆️
src/Microsoft.ML.Core/Data/ProgressReporter.cs	77.94% <0%> (+6.98%)	⬆️