[SymForce-External] Check sparse matrix condition for Jacobian and Hessian constructors

Check sparse matrix condition for Jacobian and Hessian constructors

Hi, a keen user of symforce here!

Found some code that may lead to some confusion in the future.
Suggesting a change for that.

Best,
changh95

---

- Variable `jacobian_is_sparse` should really be `kIsSparseEigenType<T>()` instead of `!kIsEigenType<T>()`
- So is for `hessian_is_sparse`

Closes #293

GitOrigin-RevId: d7ba870631c50db75fc1f24a8886ad2c98d29163

Author: changh95

symforce/opt/factor.tcc

@@ -55,7 +55,7 @@ Factor<Scalar> Factor<Scalar>::Jacobian(Functor&& func, const std::vector<Key>&
                 "JacobianMat (last argument) should be an Eigen::Matrix or Eigen::SparseMatrix");
 
   // Check sparse vs dense
-  constexpr bool is_sparse = !kIsEigenType<JacobianMat>;
+  constexpr bool is_sparse = kIsSparseEigenType<JacobianMat>;
 
   // Get dimensions
   constexpr int M = ResidualVec::RowsAtCompileTime;
@@ -113,8 +113,8 @@ Factor<Scalar> Factor<Scalar>::Hessian(Functor&& func, const std::vector<Key>& k
   static_assert(kIsEigenType<RhsVec>, "RhsVec (last argument) should be an Eigen::Matrix");
 
   // Check sparse vs dense
-  constexpr bool jacobian_is_sparse = !kIsEigenType<JacobianMat>;
-  constexpr bool hessian_is_sparse = !kIsEigenType<HessianMat>;
+  constexpr bool jacobian_is_sparse = kIsSparseEigenType<JacobianMat>;
+  constexpr bool hessian_is_sparse = kIsSparseEigenType<HessianMat>;
   static_assert(jacobian_is_sparse == hessian_is_sparse,
                 "Matrices cannot be mixed dense and sparse.");