Converted WordEmbeddingsTransform into Estimator. (#928)

* Converted WordEmbeddingsTransform into Estimator.

* Addressed reviewers' comments.

* Added documentation for public API and addressed reviewers' comments.

* Addressed reviewers' comments.

* Addressed reviewers' comments.

* Changed the estimator from trivial estimator to non-trivial estimator.

Author: zeahmed

src/Microsoft.ML.Transforms/EntryPoints/TextAnalytics.cs

@@ -150,7 +150,7 @@ public static CommonOutputs.TransformOutput WordEmbeddings(IHostEnvironment env,
             env.CheckValue(input, nameof(input));
 
             var h = EntryPointUtils.CheckArgsAndCreateHost(env, "WordEmbeddings", input);
-            var view = new WordEmbeddingsTransform(h, input, input.Data);
+            var view = WordEmbeddingsTransform.Create(h, input, input.Data);
             return new CommonOutputs.TransformOutput()
             {
                 Model = new TransformModel(h, view, input.Data),