Gradient and hessian incorrect for inverse gaussian

[stanmart]

I believe that inv_gaussian_log_rowwise_gradient_hessian is incorrect, and it should instead be

@@ -276,12 +276,12 @@ def inv_gaussian_log_rowwise_gradient_hessian(
         inv_mu = 1 / mu[i]
         inv_mu2 = inv_mu ** 2
 
-        gradient_rows_out[i] = 2 * weights[i] * (inv_mu - y[i] * inv_mu2)
-        hessian_rows_out[i] = 2 * weights[i] * (inv_mu - 2 * y[i] * inv_mu2)
+        gradient_rows_out[i] = weights[i] * (y[i] * inv_mu2 - inv_mu)
+        hessian_rows_out[i] = weights[i] * (2 * y[i] * inv_mu2 - inv_mu)

@lbittarello, let me know if you agree.

Originally posted by @stanmart in #889 (comment)

[lbittarello]

Yes, the signs should be flipped.