Adding seperable conv2d gradient test (tensorflow#513)

Author: Shashi456

Tests/TensorFlowTests/LayerTests.swift

@@ -286,6 +286,43 @@ final class LayerTests: XCTestCase {
         XCTAssertEqual(output, expected)
     }
 
+    func testSeparableConv2DGradient() {
+        let depthwiseFilter = Tensor(shape: [2, 1, 2, 2], scalars: (0..<8).map(Float.init))
+        let pointwiseFilter = Tensor(shape: [1, 1, 4, 1], scalars: (0..<4).map(Float.init))
+        let bias = Tensor<Float>([1, 1])
+        let layer = SeparableConv2D<Float>(depthwiseFilter: depthwiseFilter,
+                                           pointwiseFilter: pointwiseFilter,
+                                           bias: bias,
+                                           activation: identity,
+                                           strides: (1, 1),
+                                           padding: .same)
+        let input = Tensor(shape: [2, 1, 2, 2], scalars: (0..<8).map(Float.init))
+        let grads = gradient(at: input, layer) { $1($0).sum() }
+        // The expected value of the gradient was computed using the following Python code:
+        // ```
+        // import tensorflow as tf
+        // x = tf.reshape(tf.range(8, dtype=tf.float32), [2, 1, 2, 2])
+        // depthwiseFilter = tf.reshape(tf.range(8, dtype=tf.float32), [2, 1, 2, 2])
+        // pointwiseFilter = tf.reshape(tf.range(4, dtype=tf.float32), [1, 1, 4, 1])
+        // bias = tf.ones([2])
+        // with tf.GradientTape() as tape:
+        //     tape.watch([x, depthwiseFilter, pointwiseFilter, bias])
+        //     y = tf.math.reduce_sum(tf.nn.separable_conv2D(input,
+        //                                                   depthwiseFilter,
+        //                                                   pointwiseFilter
+        //                                                   strides=[1, 1, 1, 1],
+        //                                                   padding="SAME") + bias)
+        // print(tape.gradient(y, [x, depthwiseFilter, pointwiseFilter, bias])
+        // ```
+        XCTAssertEqual(grads.0,
+                       [[[[ 2.0, 26.0], [ 2.0, 26.0]]],
+                        [[[ 2.0, 26.0], [ 2.0, 26.0]]]])
+        XCTAssertEqual(grads.1.depthwiseFilter,
+                       [[[[ 0.0, 24.0], [64.0, 96.0]]],
+                        [[[ 0.0,  0.0], [ 0.0,  0.0]]]])
+        XCTAssertEqual(grads.1.bias, [4.0, 4.0])
+    }
+
     func testZeroPadding1D() {
         let input = Tensor<Float>(shape: [1, 3, 1], scalars: [0.0, 1.0, 2.0])
         let layer = ZeroPadding1D<Float>(padding: 2)
@@ -1241,6 +1278,7 @@ final class LayerTests: XCTestCase {
         ("testDepthwiseConv2DGradient", testDepthwiseConv2DGradient),
         ("testSeparableConv1D", testSeparableConv1D),
         ("testSeparableConv2D", testSeparableConv2D),
+        ("testSeparableConv2DGradient", testSeparableConv2DGradient),
         ("testZeroPadding1D", testZeroPadding1D),
         ("testZeroPadding1DGradient", testZeroPadding1DGradient),
         ("testZeroPadding2D", testZeroPadding2D),