Fix relation between IsSavedModel and isFrozen in DnnRetrainTransformer.SaveModel

[yamachu]

Fix: #4191

IsSavedModel returns true when loaded model is an-frozen model

machinelearning/src/Microsoft.ML.Dnn/DnnUtils.cs

Lines 137 to 143 in 1503b0a

	internal static bool IsSavedModel(IHostEnvironment env, string modelPath)
	{
	Contracts.Check(env != null, nameof(env));
	env.CheckNonWhiteSpace(modelPath, nameof(modelPath));
	FileAttributes attr = File.GetAttributes(modelPath);
	return attr.HasFlag(FileAttributes.Directory);
	}

but now isFrozen variable is set true in spite of an-frozen model.

machinelearning/src/Microsoft.ML.Dnn/DnnRetrainTransform.cs

Line 681 in 3c02da5

var isFrozen = DnnUtils.IsSavedModel(_env, _modelLocation);

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[codemzs]

Thanks @yamachu ! I recall fixing this bug in other DNN api but probably missed here. Good work investigating and fixing.