Add PartitionedFileLoader (dotnet#61)

Author: tyclintw

src/Microsoft.ML.Core/Utilities/PathUtils.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
@@ -67,13 +67,17 @@ public static string FindExistentFileOrNull(string fileName, string folderPrefix
             // 1. Search in customSearchDir.
             if (!string.IsNullOrWhiteSpace(customSearchDir)
                 && TryFindFile(fileName, folderPrefix, customSearchDir, out candidate))
-                    return candidate;
+            {
+                return candidate;
+            }
 
             // 2. Search in the path specified by the environment variable.
             var envDir = Environment.GetEnvironmentVariable(CustomSearchDirEnvVariable);
             if (!string.IsNullOrWhiteSpace(envDir)
                 && TryFindFile(fileName, folderPrefix, envDir, out candidate))
-                    return candidate;
+            {
+                return candidate;
+            }
 
             // 3. Search in the path specified by the assemblyForBasePath.
             if (assemblyForBasePath != null)

src/Microsoft.ML.Data/Commands/DataCommand.cs

@@ -396,6 +396,20 @@ public static void SaveLoader(IDataLoader loader, IFileHandle file)
             Contracts.CheckParam(file.CanWrite, nameof(file), "Must be writable");
 
             using (var stream = file.CreateWriteStream())
+            {
+                SaveLoader(loader, stream);
+            }
+        }
+
+        /// <summary>
+        /// Saves <paramref name="loader"/> to the specified <paramref name="stream"/>.
+        /// </summary>
+        public static void SaveLoader(IDataLoader loader, Stream stream)
+        {
+            Contracts.CheckValue(loader, nameof(loader));
+            Contracts.CheckValue(stream, nameof(stream));
+            Contracts.CheckParam(stream.CanWrite, nameof(stream), "Must be writable");
+
             using (var rep = RepositoryWriter.CreateNew(stream))
             {
                 ModelSaveContext.SaveModel(rep, loader, ModelFileUtils.DirDataLoaderModel);