Update x10 CUDA build instructions (#939)

* Update x10 CUDA build instructions

* Fix code style issues with swift-format

Co-authored-by: Swift for TensorFlow <ainu-bot@google.com>

Author: asuhan

README.md

@@ -2,7 +2,7 @@
 
 Get a taste of *protocol-oriented differentiable programming*.
 
-This repository hosts [Swift for TensorFlow](https://github.com/tensorflow/swift)'s deep learning library, available both as a part of Swift for TensorFlow toolchains and as a Swift package. 
+This repository hosts [Swift for TensorFlow](https://github.com/tensorflow/swift)'s deep learning library, available both as a part of Swift for TensorFlow toolchains and as a Swift package.
 
 ## Usage
 
@@ -112,6 +112,8 @@ and its argument are elided).
 If `swiftc` is not in your `PATH`, you must specify the path to it using
 `-D CMAKE_Swift_COMPILER=`.
 
+To enable CUDA support, run `export TF_NEED_CUDA=1` before the steps below.
+
 ```
 cmake -G Ninja -B out -S swift-apis
 cmake --build out

Tests/TensorFlowTests/EpochsTests.swift

@@ -158,7 +158,7 @@ final class EpochsTests: XCTestCase {
       var epochSampleCount = 0
       for batch in epochBatches {
         XCTAssertEqual(
-           batch.count, batchSize, "unexpected batch size: \(batch.count)")
+          batch.count, batchSize, "unexpected batch size: \(batch.count)")
         epochSampleCount += batch.count
       }
       let expectedDropCount = dataset.count % 64