Comment out expand in separate_terms

Author: bowenszhu

src/utils.jl

@@ -60,7 +60,8 @@ Variables in `vars` must be unique.
 function separate_terms(exprs::AbstractVector, vars, iv)
     vars = Symbolics.unwrap.(vars)
     exprs = Symbolics.unwrap.(exprs)
-    exprs = expand.(exprs)
+    # expand is helpful for separating terms but is harmful for generating effiicient runtime function
+    # exprs = expand.(exprs)
     linear_I = Int[] # row idx for sparse matrix
     linear_J = Int[] # col idx for sparse matrix
     linear_V = Float64[] # values