Update to OnnxRuntime 1.6.0 and fixed bug with sequences outputs (#5529)

* Use onnx prerelease

* Upgrade to onnx 1.6.0

* Updated docs

* Fixed problem with sequences

Author: antoniovs1029

eng/Versions.props

@@ -23,7 +23,7 @@
     <GoogleProtobufPackageVersion>3.10.1</GoogleProtobufPackageVersion>
     <LightGBMPackageVersion>2.2.3</LightGBMPackageVersion>
     <MicrosoftExtensionsPackageVersion>2.1.0</MicrosoftExtensionsPackageVersion>
-    <MicrosoftMLOnnxRuntimePackageVersion>1.5.2</MicrosoftMLOnnxRuntimePackageVersion>
+    <MicrosoftMLOnnxRuntimePackageVersion>1.6.0</MicrosoftMLOnnxRuntimePackageVersion>
     <MlNetMklDepsPackageVersion>0.0.0.9</MlNetMklDepsPackageVersion>
     <ParquetDotNetPackageVersion>2.1.3</ParquetDotNetPackageVersion>
     <SystemDrawingCommonPackageVersion>4.5.0</SystemDrawingCommonPackageVersion>

src/Microsoft.ML.OnnxTransformer/OnnxTransform.cs

@@ -768,7 +768,7 @@ public NamedOnnxValue GetNamedOnnxValue()
     /// | Does this estimator need to look at the data to train its parameters? | No |
     /// | Input column data type | Known-sized vector of <xref:System.Single> or <xref:System.Double> types |
     /// | Output column data type | As specified by the ONNX model |
-    /// | Required NuGet in addition to Microsoft.ML | Microsoft.ML.OnnxTransformer (always),  either Microsoft.ML.OnnxRuntime 1.5.2 (for CPU processing) or Microsoft.ML.OnnxRuntime.Gpu 1.5.2 (for GPU processing if GPU is available) |
+    /// | Required NuGet in addition to Microsoft.ML | Microsoft.ML.OnnxTransformer (always),  either Microsoft.ML.OnnxRuntime 1.6.0 (for CPU processing) or Microsoft.ML.OnnxRuntime.Gpu 1.6.0 (for GPU processing if GPU is available) |
     /// | Exportable to ONNX | No |
     ///
     /// To create this estimator use the following APIs:

src/Microsoft.ML.OnnxTransformer/OnnxTypeParser.cs

@@ -267,7 +267,13 @@ private class CastHelper
 
             public static IEnumerable<TDst> CastOnnxSequenceToIEnumerable<TSrc, TDst>(IEnumerable<TSrc> o, Func<TSrc, object> caster)
             {
-                return o.Select(v => (TDst)caster(v));
+                // Since now we're disposing the NamedOnnxValue objects
+                // after running inference on each output, we need
+                // to copy (enumerate) the output through ".ToList()"
+                // else, if our users try the keep the past sequence
+                // outputs of their OnnxTransformer, they would
+                // end up with empty sequences.
+                return o.Select(v => (TDst)caster(v)).ToList();
             }
         }