How to use the nbsite.gallery.thumbnailer.execute function in nbsite

retcode = 0
                    if not retcode:
                        with open(thumb_path, 'wb') as thumb_f:
                            thumb_f.write(thumb_req.content)

                # Generate thumbnail
                if not retcode or only_use_existing:
                    pass
                elif extension == 'ipynb':
                    verb = 'Successfully generated'
                    print('getting thumbnail code for %s' % os.path.abspath(f))
                    print('Path exists %s' % os.path.exists(os.path.abspath(f)))
                    code = notebook_thumbnail(os.path.abspath(f), dest_dir)
                    code = script_prefix + code
                    my_env = os.environ.copy()
                    retcode = execute(code.encode('utf8'), env=my_env, cwd=os.path.split(f)[0])
                else:
                    retcode = 1

                if retcode:
                    logger.info('%s thumbnail export failed' % basename)
                    if extension == 'py':
                        continue
                    thumb_prefix = '_'.join([pre for pre in (section, backend) if pre])
                    backend_str = backend+'_' if backend else '' 
                    if thumb_prefix:
                        thumb_prefix += '_'
                    this_entry = THUMBNAIL_TEMPLATE.format(
                        backend=backend_str, prefix=thumb_prefix, thumbnail=logo_path,
                        ref_name=basename, label=basename.replace('_', ' ').title())
                else:
                    logger.info('%s %s thumbnail' % (verb, basename))