Mention and test inputs can be > 1D in hessian_vector_prod

[ricardoV94]

I don't think there is any reason to require wrt and p to be 1d. As long as wrt and p have the same shape(s) everything should be fine.

Maybe we can add a test for that and remove the "1-dimensional" from the docs?

Originally posted by @aseyboldt in #858 (review)