WASHINGTON/JAKARTA (Reuters) -U.S. President Joe Biden is unlikely to attend summits with Southeast Asian leaders in Jakarta next month, multiple sources told Reuters, an absence that would raise questions about U.S. commitment to a region vital in its efforts to push back against China's expanding influence. Biden is due to attend a Sept. 9-10 G20 summit in India and said on Tuesday he plans to visit an important emerging Southeast Asian partner Vietnam "shortly" to upgrade ties, but his presence at the Sept. 4-7 summits of the 10-nation Association of Southeast Asian Nations (...