I rented a movie on my iPad but there isn't enough space on it to download the movie so now I'm trying to either watch it on my AppleTV or laptop through iTunes, but I can't seem to find how to do that. Is it not possible?
No, it is not possible.
See this Apple Support Document for more information:
You'll have to expand the section on "Movie rentals on iPad" to see the above quote.
Your only option is to free up enough space to watch it on your iPad.