rebuild resnet block based approach (#170)

Author: brettkoonce

ResNet/Helpers.swift

@@ -0,0 +1,51 @@
+// Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// TODO: Remove this when it's moved to the standard library.
+extension Array where Element: Differentiable {
+    @differentiable(wrt: (self, initialResult), vjp: reduceDerivative)
+    func differentiableReduce<Result: Differentiable>(
+        _ initialResult: Result,
+        _ nextPartialResult: @differentiable (Result, Element) -> Result
+    ) -> Result {
+        return reduce(initialResult, nextPartialResult)
+    }
+
+    func reduceDerivative<Result: Differentiable>(
+        _ initialResult: Result,
+        _ nextPartialResult: @differentiable (Result, Element) -> Result
+    ) -> (Result, (Result.TangentVector) -> (Array.TangentVector, Result.TangentVector)) {
+        var pullbacks: [(Result.TangentVector)
+            -> (Result.TangentVector, Element.TangentVector)] = []
+        let count = self.count
+        pullbacks.reserveCapacity(count)
+        var result = initialResult
+        for element in self {
+            let (y, pb) = Swift.valueWithPullback(at: result, element, in: nextPartialResult)
+            result = y
+            pullbacks.append(pb)
+        }
+        return (value: result, pullback: { cotangent in
+            var resultCotangent = cotangent
+            var elementCotangents = TangentVector([])
+            elementCotangents.base.reserveCapacity(count)
+            for pullback in pullbacks.reversed() {
+                let (newResultCotangent, elementCotangent) = pullback(resultCotangent)
+                resultCotangent = newResultCotangent
+                elementCotangents.base.append(elementCotangent)
+            }
+            return (TangentVector(elementCotangents.base.reversed()), resultCotangent)
+        })
+    }
+}