Compute Gramians of Jacobians by combining vjp and jvp #149

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.