Add @Freezable property wrapper.

`@Freezable` wraps differentiable values and provides toggleable
trainability via the `isFrozen` property and `freeze`/`unfreeze`
mutating methods.

When `isFrozen` is true, accesses to `value` have a derivative of zero.

Author: dan-zheng

Tests/TensorFlowTests/FreezableTests.swift

@@ -0,0 +1,76 @@
+// 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.
+
+import XCTest
+@testable import TensorFlow
+
+final class FreezableTests: XCTestCase {
+    func testFreezableParameters() {
+        // A dense layer with freezable properties.
+        struct FreezableDense : Layer {
+            @Freezable var weight: Tensor<Float>
+            @Freezable var bias: Tensor<Float>
+
+            init(weight: Tensor<Float>, bias: Tensor<Float>) {
+                // Require scalar weight and bias for simplicity.
+                precondition(weight.isScalar)
+                precondition(bias.isScalar)
+                self.weight = weight
+                self.bias = bias
+            }
+
+            @differentiable
+            func callAsFunction(_ input: Tensor<Float>) -> Tensor<Float> {
+                return input * weight + bias
+            }
+        }
+
+        var dense = FreezableDense(weight: Tensor(2), bias: Tensor(3))
+        let x = Tensor<Float>(4)
+        do {
+            let (value, gradient) = valueWithGradient(at: dense, x) { dense, x in dense(x) }
+            XCTAssertEqual(Tensor(11), value)
+            // The gradient of `dense.weight` should be non-zero.
+            XCTAssertEqual(FreezableDense.TangentVector(_weight: Tensor(4), _bias: Tensor(1)),
+                           gradient.0)
+            XCTAssertEqual(Tensor(2), gradient.1)
+        }
+
+        // Freeze derivatives for `dense.weight`.
+        dense.$weight.freeze()
+        do {
+            let (value, gradient) = valueWithGradient(at: dense, x) { dense, x in dense(x) }
+            // The gradient of `dense.weight` should now be zero.
+            XCTAssertEqual(Tensor(11), value)
+            XCTAssertEqual(FreezableDense.TangentVector(_weight: Tensor(0), _bias: Tensor(1)),
+                           gradient.0)
+            XCTAssertEqual(Tensor(2), gradient.1)
+        }
+
+        // Unfreeze derivatives for `dense.weight`.
+        dense.$weight.unfreeze()
+        do {
+            let (value, gradient) = valueWithGradient(at: dense, x) { dense, x in dense(x) }
+            XCTAssertEqual(Tensor(11), value)
+            // The gradient of `dense.weight` should now be non-zero.
+            XCTAssertEqual(FreezableDense.TangentVector(_weight: Tensor(4), _bias: Tensor(1)),
+                           gradient.0)
+            XCTAssertEqual(Tensor(2), gradient.1)
+        }
+    }
+
+    static var allTests = [
+        ("testFreezableParameters", testFreezableParameters),
+    ]
+}