By Trevor Hunnicutt, Lisandra Paraguassu and Matt Spetalnick LOS ANGELES (Reuters) – U.S. President Joe Biden laid out his pitch to leaders from the Americas for a new economic partnership at the start of a regional summit on Thursday, even as he faced open rebuke over the exclusion of Venezuela, Cuba and Nicaragua. Speaking at the opening session of the Summit of the Americas in Los Angeles, Biden said his administration was committed to helping Latin America and the Caribbean recover from the COVID-19 pandemic, tackle irregular migration and improve living standards. But he quickly faced sha…