Paytm php sdk integration PaymentDetailBuilder Class not found - php

Question

Fatal error: Uncaught Error: Class 'PaymentDetailBuilder' not found
initial code is like this:
require_once('vendor/autoload.php');
use paytmpg\merchant\models\PaymentDetail\PaymentDetailBuilder;
use paytmpg\merchant\models\PaymentStatusDetail\PaymentStatusDetailBuilder;
use paytmpg\merchant\models\RefundDetail\RefundDetailBuilder;
use paytmpg\merchant\models\RefundStatusDetail\RefundStatusDetailBuilder;
// For Staging
$environment = LibraryConstants::STAGING_ENVIRONMENT;
// For Production
// $environment = LibraryConstants::PRODUCTION_ENVIRONMENT;
// Find your mid, key, website in your Paytm Dashboard at https://dashboard.paytm.com/next/apikeys
$mid = MERCHANT_MID;
$key = MERCHANT_KEY;
$website = MERCHANT_WEBSITE;
$client_id = MERCHANT_CLIENT_ID;
$callbackUrl = MERCHANT_CALLBACK_URL;
MerchantProperties::setCallbackUrl($callbackUrl);
MerchantProperties::initialize($environment, $mid, $key, $client_id, $website);
// If you want to add log file to your project, use below code
// Config::$monologName = '[PAYTM]';
// Config::$monologLevel = MonologLogger::INFO;
// Config::$monologLogfile = 'file.log';
$channelId = EChannelId::WEB;
$orderId = "ORDER_ID".rand(0,99); // post order id
$txnAmount = Money::constructWithCurrencyAndValue(EnumCurrency::INR, "11.00"); // post amount to be paid
$userInfo = new UserInfo("CUSTID_002");
$userInfo->setAddress("CUSTOMER_ADDRESS");
$userInfo->setEmail("CUSTOMER_EMAIL_ID");
$userInfo->setFirstName("CUSTOMER_FIRST_NAME");
$userInfo->setLastName("CUSTOMER_LAST_NAME");
$userInfo->setMobile("CUSTOMER_MOBILE_NO");
$userInfo->setPincode("CUSTOMER_PINCODE");
$paymentDetailBuilder = new PaymentDetailBuilder($channelId, $orderId, $txnAmount, $userInfo);
$paymentDetail = $paymentDetailBuilder->build();
$response = Payment::createTxnToken($paymentDetail);
print_r($response);
I have copied from => https://developer.paytm.com/docs/server-sdk/php/?ref=serverSdk#SDKCode;