add a separate file for fwd decl of broadcast array

Author: SteveBronder

stan/math/fwd/functor/broadcast_array.hpp

@@ -3,15 +3,13 @@
 
 #include <stan/math/prim/fun/Eigen.hpp>
 #include <stan/math/fwd/fun/sum.hpp>
+#include <stan/math/prim/functor/broadcast_array_fwd.hpp>
 #include <stan/math/prim/meta.hpp>
 
 namespace stan {
 namespace math {
 namespace internal {
 
-template <typename T, typename Enable = void>
-class broadcast_array;
-
 template <typename T>
 class broadcast_array<T, require_st_fvar<T>> {
  private:

stan/math/prim/functor/broadcast_array.hpp

@@ -1,6 +1,7 @@
 #ifndef STAN_MATH_PRIM_META_BROADCAST_ARRAY_HPP
 #define STAN_MATH_PRIM_META_BROADCAST_ARRAY_HPP
 
+#include <stan/math/prim/functor/broadcast_array_fwd.hpp>
 #include <stan/math/prim/meta/is_eigen.hpp>
 #include <stan/math/prim/meta/is_var_or_arithmetic.hpp>
 #include <stan/math/prim/meta/promote_scalar_type.hpp>
@@ -14,9 +15,6 @@ namespace stan {
 namespace math {
 namespace internal {
 
-template <typename T, typename Enable = void>
-class broadcast_array;
-
 template <typename T>
 class broadcast_array<T, require_t<is_var_or_arithmetic<T>>> {
  private:

stan/math/prim/functor/broadcast_array_fwd.hpp

@@ -0,0 +1,14 @@
+#ifndef STAN_MATH_PRIM_META_BROADCAST_ARRAY_FWD_HPP
+#define STAN_MATH_PRIM_META_BROADCAST_ARRAY_FWD_HPP
+
+namespace stan {
+namespace math {
+namespace internal {
+
+template <typename T, typename Enable = void>
+class broadcast_array;
+
+}  // namespace internal
+}  // namespace math
+}  // namespace stan
+#endif