fetch all post from wp multisite network and sort all the post in descending order on the basis of “Created by ” term

So you first want to order them by author and then by date for every author?
You should use the orderby argument in your query, where you can pass an array.

$custom_query_args = array(
  'post_type'  => 'post',
  'meta_key'   => 'xyz',
  'meta_value' => 'yes',
  'orderby'    => array( 'author' => 'ASC', 'date' => 'DESC' )
  );

This should be something like what you want, otherwise look at the examples for the arguments in WP_Query.