I'm looking for one as well. Currently using JeFit but they've stated Apple Watch is not a priority as they're presently focused on Android development. So far, the only app I've found for weight training that's Apple Watch compatible is Reps & Sets.
Apparently they did a Kickstarter to fund the watch app but it didn't get any traction. I hope they try again - Reps & Sets is the best weight training tracking app I've found.