WASHINGTON - Carly Fiorina, a formal rival of Donald Trump for the 2016 Republican presidential nomination, plans to cast her vote for Democrat Joe Biden in November."I've been very clear that I can't support Donald Trump," Fiorina told The Atlantic magazine in an interview published on Thursday.Fiorina, the former Hewlett-Packard Co chief executive, fell in line behind Trump like many Republicans in his 2016 race against Democrat Hillary Clinton. Since then, she has become increasingly critical and last year called for his impeachment."I am encouraged that Joe Biden is a person of humility an...