Throw exception in ImageClassificationTrainer when dataset contains only 1 class

[antoniovs1029]

Throw exception in ImageClassificationTrainer when dataset contains only 1 class

Fixes #4660

[codecov]

Codecov Report

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The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4662   +/-   ##
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  Coverage          ?   75.83%           
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  Files             ?      951           
  Lines             ?   172306           
  Branches          ?    18588           
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  Hits              ?   130664           
  Misses            ?    36466           
  Partials          ?     5176

Flag	Coverage Δ
#Debug	75.83% <100%> (?)
#production	71.43% <100%> (?)
#test	90.6% <ø> (?)

Impacted Files	Coverage Δ
.../Microsoft.ML.Vision/ImageClassificationTrainer.cs	88.98% <100%> (ø)

[harishsk]

🕐

[harishsk]

[codemzs]

[natke]

See my reply to the discussion on message clarity and consistency

[antoniovs1029]

I've implemented the message that @natke suggested.

I had to use the "var msg" to store the string, because I couldn't use string interpolation inside the CheckParam() argument, because of this:

machinelearning/tools-local/Microsoft.ML.InternalCodeAnalyzer/Descriptions.Designer.cs

Lines 64 to 71 in fa75f4f

	/// <summary>
	/// Looks up a localized string similar to Since C# has no concept of lazy evaluation of parameters, we prefer Contracts.Check&apos;s message arguments to not involve string formatting, or other complex operations, since such operations will happen always, whether the check fails or not. If you want to have detailed messages that&apos;s great, but use Contracts.Except instead. That is instead of something like &apos;Check(c, msg)&apos;, prefer &apos;if (!c) throw Except(msg)&apos;..
	/// </summary>
	internal static string ContractsCheckMessageNotLiteralOrIdentifier {
	get {
	return ResourceManager.GetString("ContractsCheckMessageNotLiteralOrIdentifier", resourceCulture);
	}
	}

So the suggestion there is to use Contracts.Except() instead of Contracts.Param() if I want to use string interpolation. Problem is Contracts.Except() throws an InvalidOperationException (which @justinormont and @harishsk had suggested I shouldn't use). So I guess it's best to simply create the "msg" string before passing it to the Contracts.CheckParam()...