May 3 (Reuters) – U.S. microchip export controls imposed last year to freeze China's development of supercomputers used to develop nuclear weapons and artificial-intelligence systems like ChatGPT are having only minimal effects on China's tech sector. The rules restricted shipments of Nvidia Corp (NVDA.O) and Advanced Micro Devices Inc (AMD.O) chips that have become the global technology industry's standard for developing chatbots and other AI systems. But Nvidia has created variants of its chips for the Chinese market that are slowed down to meet U.S. rules. Industry experts told Reuters the newest one – the Nvidia H800, announced in March…