Use updated APIs (• and toAccelerator). (#17)

- `Tensor.dot(_:)` and `⊗` were deprecated in favor of `matmul(_:_:)` and `•`.
- `toDevice` was renamed to `toAccelerator`.

Author: dan-zheng

MNIST/MNIST.swift

@@ -75,17 +75,17 @@ func main() {
     var i: Int32 = 0
     repeat {
         // Forward pass.
-        let z1 = images ⊗ w1 + b1
+        let z1 = images • w1 + b1
         let h1 = sigmoid(z1)
-        let z2 = h1 ⊗ w2 + b2
+        let z2 = h1 • w2 + b2
         let predictions = sigmoid(z2)
 
         // Backward pass.
         let dz2 = (predictions - labels) / batchSize
-        let dw2 = h1.transposed(withPermutations: 1, 0) ⊗ dz2
+        let dw2 = h1.transposed(withPermutations: 1, 0) • dz2
         let db2 = dz2.sum(squeezingAxes: 0)
-        let dz1 = dz2.dot(w2.transposed(withPermutations: 1, 0)) * h1 * (1 - h1)
-        let dw1 = images.transposed(withPermutations: 1, 0) ⊗ dz1
+        let dz1 = matmul(dz2, w2.transposed(withPermutations: 1, 0)) * h1 * (1 - h1)
+        let dw1 = images.transposed(withPermutations: 1, 0) • dz1
         let db1 = dz1.sum(squeezingAxes: 0)
 
         // Gradient descent.