Walmart, the world's biggest retailer WMT.N gained 1.5% after raising its full-year earnings forecast as additional stimulus checks increased spending for apparel and electronics. By Medha Singh May 18 (Reuters) - U.S. stock index futures climbed on Tuesday after better-than-expected results from Walmart and Home Depot signaled strength in consumer demand against the backdrop of rising prices. Wall Street's main indexes fell on Monday on fears that an overheating economy could prompt the Federal Reserve to rein in its monetary support following a spike in volatility last week after strong inflation readings.