The Inca Trail treks are situated in Cusco region, in the Southern Peruvian Andes. So the usual weather expected on the Inca Trail is clearly marked into two different seasons: dry and wet, from May to October is considered the dry season with a lower chance to get rainy days, mostly sunny and hot but cold at night especially in winter time. From November to April is the wet season, you can expect mainly heave rainy days, thunder storms, cloudy and normally not the best visibility on the mountains.
For the classic Inca Trail hikers we highly recommend them to bring a good four season sleeping bag or it could be hired here in Cusco, also a change of clothes for sunny days and rain gear. The area of the short Inca Trail is in the cloud forest so the weather is warmer and humid. Only at Machu Picchu you can expect very changeable weather, usually unpredictable, but still sunny days at winter time (June to August).
If you are hiking the Short Inca Trail to Machu Picchu sleeping bags are not required because you will stay at the hotel in Aguas Calientes, but rain gear is highly advised and especially in the wet season (November to April).