As Trump's tariff threat shakes up AI stocks like Nvidia, investors are asking whether this once unstoppable stock is now a brilliant bargain or a falling knife that not even Warren Buffett would touch.Nvidia (NVDA -0.62%) stock is down roughly 25% from its all-time high, rattled by Trump's latest tariff threats and the rise of artificial intelligence (AI) competitors like DeepSeek. But is this pullback a warning sign or a rare opportunity to invest in a brilliant, unstoppable tech giant before it skyrocket ...