Tensor extensions

[KsenijaS]

Output tensors from graph are being copied from unamanaged to managed memory and every time a new buffer is created. Performance will be better if we reuse buffer when it's possible.
Improve performance by replacing ToArray method in Tensor class with extension methods: ToScalar, ToSpan and ToArray

[eerhardt]

using System;

copyright header is needed on all code #Resolved

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Refers to: src/Microsoft.ML.Dnn/TensorTypeExtensions.cs:1 in 0b2c97e. [](commit_id = 0b2c97e, deletion_comment = False)

[codecov]

Codecov Report

Merging #4260 into master will decrease coverage by <.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4260      +/-   ##
==========================================
- Coverage   74.48%   74.48%   -0.01%     
==========================================
  Files         877      878       +1     
  Lines      153663   153688      +25     
  Branches    16828    16830       +2     
==========================================
+ Hits       114457   114469      +12     
- Misses      34474    34479       +5     
- Partials     4732     4740       +8

Flag	Coverage Δ
#Debug	74.48% <71.42%> (-0.01%)	⬇️
#production	70.07% <71.42%> (-0.01%)	⬇️
#test	89.48% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/Microsoft.ML.Dnn/DnnUtils.cs	66.08% <ø> (-0.87%)	⬇️
src/Microsoft.ML.TensorFlow/TensorflowTransform.cs	74.07% <50%> (+0.12%)	⬆️
...c/Microsoft.ML.Dnn/ImageClassificationTransform.cs	77.31% <58.82%> (-0.32%)	⬇️
src/Microsoft.ML.Dnn/DnnRetrainTransform.cs	57.26% <75%> (+0.1%)	⬆️
src/Microsoft.ML.Dnn/TensorTypeExtensions.cs	81.81% <81.81%> (ø)
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	79.83% <0%> (-6.73%)	⬇️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	77.96% <0%> (-5.94%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	73.6% <0%> (-0.33%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	84.7% <0%> (-0.21%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.1% <0%> (-0.16%)	⬇️
... and 4 more

[Nucs]

After this PR will be merged, If needed - I'll add performant support for conversion between types instead of throwing NotSupportedException when types do not match. #Resolved

[eerhardt]

Just 2 comments. After those are addressed I believe this can be merged. Nice work.

[codemzs]