そのままコピーして使用可能。
１：前カメラを起動
２：顔を認証
３：顔に四角い枠を表示

import UIKit
import AVFoundation

class ViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate {
    
    var faceTracker: FaceTracker!

    var rectView = UIView()
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        rectView.layer.borderWidth = 2 //四角い枠
        faceTracker = FaceTracker(view: view, findface: {arr in
            let rect = arr[0] //一番の顔だけ使う
            print(rect)
            self.rectView.frame = rect //四角い枠を顔の位置に移動する
        })
        view.addSubview(rectView)
    }
}


class FaceTracker: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {
    
    let captureSession = AVCaptureSession()
    var videoDevice = AVCaptureDevice.default(.builtInTrueDepthCamera, for: .video, position: .front)
    let audioDevice = AVCaptureDevice.default(for: .audio)

    var videoOutput = AVCaptureVideoDataOutput()
    var view: UIView
    private var findface: (_ arr: Array<CGRect>) -> Void
    
    required init(view: UIView, findface: @escaping (_ arr: Array<CGRect>) -> Void) {
        self.view = view
        self.findface = findface
        super.init()
        initialize()
    }

    func initialize() {
        
        // 各デバイスの登録
        let videoInput = try! AVCaptureDeviceInput(device: videoDevice!)
        captureSession.addInput(videoInput)
        
        let audioInput = try! AVCaptureDeviceInput(device: audioDevice!)
        captureSession.addInput(audioInput)

        videoOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as AnyHashable as! String : Int(kCVPixelFormatType_32BGRA)]

        // フレーム毎に呼び出すデリゲート登録
        let queue = DispatchQueue(label: "myqueue", attributes: .concurrent)
        videoOutput.setSampleBufferDelegate(self, queue: queue)
        videoOutput.alwaysDiscardsLateVideoFrames = true

        captureSession.addOutput(videoOutput)

        let videoLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        videoLayer.frame = view.bounds
        videoLayer.videoGravity = .resizeAspectFill
        
        view.layer.addSublayer(videoLayer)

        //カメラ向き
        for c in videoOutput.connections {
            if c.isVideoOrientationSupported {
                c.videoOrientation = .portrait
            }
        }
        captureSession.startRunning()
    }

    func imageFromSampleBuffer(sampleBuffer: CMSampleBuffer) -> UIImage {
        //バッファーをUIImageに変換
        let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)!
        CVPixelBufferLockBaseAddress(imageBuffer, CVPixelBufferLockFlags(rawValue: 0))
        let context = CGContext(data: CVPixelBufferGetBaseAddressOfPlane(imageBuffer, 0),
                                width: CVPixelBufferGetWidth(imageBuffer),
                                height: CVPixelBufferGetHeight(imageBuffer),
                                bitsPerComponent: 8,
                                bytesPerRow: CVPixelBufferGetBytesPerRow(imageBuffer),
                                space: CGColorSpaceCreateDeviceRGB(),
                                bitmapInfo: CGBitmapInfo.byteOrder32Little.rawValue | CGImageAlphaInfo.premultipliedFirst.rawValue)
        let imageRef = context!.makeImage()
        CVPixelBufferUnlockBaseAddress(imageBuffer, CVPixelBufferLockFlags(rawValue: 0))
        return UIImage(cgImage: imageRef!)
    }

    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        
        DispatchQueue.main.sync(execute: {

            //バッファーをUIImageに変換
            let image = self.imageFromSampleBuffer(sampleBuffer: sampleBuffer)
            let ciimage = CIImage(image: image)!

            let detector = CIDetector(ofType: CIDetectorTypeFace, context: nil, options: [CIDetectorAccuracy: CIDetectorAccuracyLow])!
            let faces = detector.features(in: ciimage)
            
            if faces.count != 0 {
                var rects = Array<CGRect>()
                for face in faces {

                    // 座標変換
                    var faceR: CGRect = (face as AnyObject).bounds
                    let widthPer = view.bounds.width/image.size.width
                    let heightPer = view.bounds.height/image.size.height

                    // CoreImageは左下に原点があるので揃える
                    faceR.origin.y = image.size.height - faceR.origin.y - faceR.size.height

                    // 倍率変換
                    faceR.origin.x = faceR.origin.x * widthPer
                    faceR.origin.y = faceR.origin.y * heightPer
                    faceR.size.width = faceR.size.width * widthPer
                    faceR.size.height = faceR.size.height * heightPer

                    rects.append(faceR)
                }
                findface(rects)
            }
        })
    }
}
参考ページ
[コピペで使える]swift3/swift4/swift5でリアルタイム顔認識をする方法