在override的AVVideoCompositing协议方法中使用autoreleasepool来管理处理中产生的CIImage，避免内存泄漏。

示例代码如下：

class CustomCompositor: NSObject, AVVideoCompositing {

    func startRequest(_ request: AVAsynchronousVideoCompositionRequest) {

        // Create a block which will be later used for asynchronous processing
        request.finish(composedVideoFrame: CIImage(), completionHandler: { [weak self] in
            self?.cleanup()
        })
        
        // Do some heavy image processing with CIImage
        let sourcePixelBuffer = request.sourceFrame(byTrackID: 1)!
        let sourceImage = CIImage(cvPixelBuffer: sourcePixelBuffer)
        let outputImage = // Perform some CIImage processing
        
        // Use an autoreleasepool to manage memory created during the image processing
        autoreleasepool {
            // Create a new pixel buffer to return
            var pixelBuffer: CVPixelBuffer?
            CVPixelBufferCreate(nil, request.renderSize.width,
                                request.renderSize.height, kCVPixelFormatType_32BGRA,
                                nil, &pixelBuffer)

            // Render the output image to the new pixel buffer
            CIContext().render(outputImage, to: pixelBuffer!)

            // Finish the asynchronous processing by passing the final composed frame to the request
            let finalImage = CIImage(cvPixelBuffer: pixelBuffer!)
            request.finish(composedVideoFrame: finalImage)
        }
    }

    func cleanup() {
        // Cleanup any remaining resources here
    }

}