ChatGPT-maker OpenAI confidentially filed for a U.S. initial public offering, joining rival Anthropic in a push toward the stock market. June 8 (Reuters) - ChatGPT-maker OpenAI confidentially filed for a U.S. initial public offering on Monday, joining rival Anthropic in a push toward the stock market as investors seek exposure to the artificial intelligence boom. OpenAI did not disclose the size or terms of the offering and said a timeline has not yet been determined. “It may be a while because