GENEVA/BRUSSELS (Reuters) - The head of the World Trade Organization will announce his early departure on Thursday more than a year before the end of his term, Geneva-based sources said, a surprise move at a time when the agency's mechanism to resolve disputes has been paralysed.Brazilian Roberto Azevedo, 62, has been director-general since 2013 and is serving a second term due to conclude at the end of August next year. He has summoned a virtual meeting to inform national members on Thursday..