Add average, multiply and stack modes for BiRNNs

Sources/TensorFlow/Layers/Recurrent.swift

@@ -211,7 +211,7 @@ public struct LSTMCell<Scalar: TensorFlowFloatingPoint>: RecurrentLayerCell {
     /// Concatenates two values.
     @differentiable
     public static func concatenate(_ lhs: Self, _ rhs: Self) -> Self {
-      // TODO: Remove workaround after https://github.com/tensorflow/swift-apis/issues/1087 is fixed.
+      // TODO(TF-1005): Remove workaround for differenting concatenated.
       let concatCell = lhs.cell.concatenated(with: rhs.cell, alongAxis: -1)
       let concatHidden = lhs.hidden.concatenated(with: rhs.hidden, alongAxis: -1)
       let cell = concatCell.withDerivative { [shape = concatCell.shape] in
@@ -228,6 +228,33 @@ public struct LSTMCell<Scalar: TensorFlowFloatingPoint>: RecurrentLayerCell {
     public static func sum(_ lhs: Self, _ rhs: Self) -> Self {
       Self(cell: lhs.cell + rhs.cell, hidden: lhs.hidden + rhs.hidden)
     }
+
+    /// Averages two values.
+    @differentiable
+    public static func average(_ lhs: Self, _ rhs: Self) -> Self {
+      Self(cell: (lhs.cell + rhs.cell) / 2, hidden: (lhs.hidden + rhs.hidden) / 2)
+    }
+
+    /// Multiplies two values.
+    @differentiable
+    public static func multiply(_ lhs: Self, _ rhs: Self) -> Self {
+      Self(cell: lhs.cell * rhs.cell, hidden: lhs.hidden * rhs.hidden)
+    }
+
+    /// Stack two values.
+    @differentiable
+    public static func stack(_ lhs: Self, _ rhs: Self) -> Self {
+      // TODO(TF-1005): Remove workaround for differenting stacking.
+      let stackCell = Tensor(stacking: [lhs.cell, rhs.cell])
+      let stackHidden = Tensor(stacking: [lhs.hidden, rhs.hidden])
+      let cell = stackCell.withDerivative { [shape = stackCell.shape] in
+          if $0 == Tensor(0) { $0 = Tensor(zeros: shape) }
+      }
+      let hidden = stackHidden.withDerivative { [shape = stackHidden.shape] in
+          if $0 == Tensor(0) { $0 = Tensor(zeros: shape) }
+      }
+      return Self(cell: cell, hidden: hidden)
+    }
   }
 
   /// Returns a zero-valued state with shape compatible with the provided input.
@@ -455,13 +482,25 @@ public protocol Mergeable: Differentiable, AdditiveArithmetic {
   /// `Mergeable` (SR-13229).
   @differentiable
   static func sum(_ lhs: Self, _ rhs: Self) -> Self
+
+  /// Averages two values.
+  @differentiable
+  static func average(_ lhs: Self, _ rhs: Self) -> Self
+
+  /// Multiplies two values.
+  @differentiable
+  static func multiply(_ lhs: Self, _ rhs: Self) -> Self
+
+  /// Stack two values.
+  @differentiable
+  static func stack(_ lhs: Self, _ rhs: Self) -> Self
 }
 
 extension Tensor: Mergeable where Scalar: TensorFlowFloatingPoint {
   /// Concatenates two tensors along last axis.
   @differentiable
   public static func concatenate(_ lhs: Tensor, _ rhs: Tensor) -> Tensor {
-    // TODO: Remove workaround after https://github.com/tensorflow/swift-apis/issues/1087 is fixed.
+    // TODO(TF-1005): Remove workaround for differenting concatenated.
     let concat = lhs.concatenated(with: rhs, alongAxis: -1)
     return concat.withDerivative { [shape = concat.shape] in
         if $0 == Tensor(0) { $0 = Tensor(zeros: shape) }
@@ -473,6 +512,28 @@ extension Tensor: Mergeable where Scalar: TensorFlowFloatingPoint {
   public static func sum(_ lhs: Tensor, _ rhs: Tensor) -> Tensor {
     lhs + rhs
   }
+
+  /// Averages two values.
+  @differentiable
+  public static func average(_ lhs: Tensor, _ rhs: Tensor) -> Tensor {
+    (lhs + rhs) / 2
+  }
+
+  /// Multiplies two values.
+  @differentiable
+  public static func multiply(_ lhs: Tensor, _ rhs: Tensor) -> Tensor {
+    lhs * rhs
+  }
+
+  /// Stack two values.
+  @differentiable
+  public static func stack(_ lhs: Tensor, _ rhs: Tensor) -> Tensor {
+    // TODO(TF-1005): Remove workaround for differenting stacking.
+    let stack = Tensor(stacking: [lhs, rhs])
+    return stack.withDerivative { [shape = stack.shape] in
+        if $0 == Tensor(0) { $0 = Tensor(zeros: shape) }
+    }
+  }
 }
 
 /// Concatenates two values.
@@ -493,6 +554,33 @@ public func sum<T: Mergeable>(
   T.sum(first, second)
 }
 
+/// Averages two values.
+@differentiable
+public func average<T: Mergeable>(
+  _ first: T,
+  _ second: T
+) -> T {
+  T.average(first, second)
+}
+
+/// Multiplies two values.
+@differentiable
+public func multiply<T: Mergeable>(
+  _ first: T,
+  _ second: T
+) -> T {
+  T.multiply(first, second)
+}
+
+/// Stack two values.
+@differentiable
+public func stack<T: Mergeable>(
+  _ first: T,
+  _ second: T
+) -> T {
+  T.stack(first, second)
+}
+
 public struct BidirectionalRecurrentLayer<Cell: RecurrentLayerCell>: Layer
 where Cell.TimeStepOutput: Mergeable {
   public typealias Input = [Cell.TimeStepInput]