Compute Gramians of Jacobians by combining `vjp` and `jvp`

If $J$ is the Jacobian of some $f$ at some $x$, then we compute the jacobian using `vmap` on `jvp` or `vjp`, then we compute the Gramian by computing $JJ^\top$.
What we could do is to compose `vjp` and `jvp` and use `vmap` on that to compute the Gramian directly without conputing the full Jacobian. This should be much faster.