There are solo drills and movement exercises which can be very helpful for grapplers to practice on their own. However I don’t think it’s possible to develop a high level of application skill without training partners.I think it's easier to practice striking arts than grappling arts on your own, too. It's a lot easier to imagine the striking than the grappling. In a sparring situation, there's a lot less contact in striking than grappling, making it less unrealistic when you practice on your own.