SAN FRANCISCO - Venture capital firm Andreessen Horowitz is seeking to raise about $20 billion in what will be the largest fund in its history, to capitalize on global investors' interest in backing U.S. artificial intelligence companies, sources told Reuters. The tech investment firm, known informally as a16z, has told limited partners that the fund will be dedicated to growth-stage investments in AI companies and draw upon global investors keen on investing in American companies, the sources