By Jose Torres and Lizbeth Diaz TAPACHULA (Reuters) – Several thousand migrants, many from Venezuela, set off from southern Mexico early Monday aiming to reach the United States, timing their journey to coincide with the Summit of the Americas in Los Angeles this week. Migration activists said the group could be one of the region’s largest migrant caravans in recent years. At least 6,000 people, according to Reuters witnesses, left the border city of Tapachula. Mexico’s National Institute for Migration did not provide an estimate of the group’s size and provided no additional comment on the ca…