Add get_transformed_data_json to model_base

src/stan/model/model_base.hpp

@@ -179,6 +179,15 @@ class model_base : public prob_grad {
                                          bool include_tparams = true,
                                          bool include_gqs = true) const = 0;
 
+  /**
+   * Return a string containing the transformed data computed by the model at
+   * construction as JSON string.
+   * The format of the string is the same as the expected input format for the
+   * `stan::json` input methods, i.e. complex numbers are rendered as a 2-list,
+   * tuples as objects with integer keys counting from 1
+   */
+  virtual std::string get_transformed_data_json() const = 0;
+
   /**
    * Return the log density for the specified unconstrained
    * parameters, without Jacobian and with normalizing constants for

src/test/unit/io/random_var_context_test.cpp

@@ -71,6 +71,9 @@ class mock_throwing_model_in_write_array : public stan::model::prob_grad {
       param_names__.push_back(param_name.str());
     }
   }
+
+  std::string get_transformed_data_json() const { return std::string("{}"); }
+
   template <typename RNG>
   void write_array(RNG& base_rng__, std::vector<double>& params_r__,
                    std::vector<int>& params_i__, std::vector<double>& vars__,

src/test/unit/model/model_base_crtp_test.cpp

@@ -34,6 +34,10 @@ struct mock_model : public stan::model::model_base_crtp<mock_model> {
                                  bool include_tparams,
                                  bool include_gqs) const override {}
 
+  std::string get_transformed_data_json() const override {
+    return std::string("{}");
+  }
+
   template <bool propto, bool jacobian, typename T>
   T log_prob(Eigen::Matrix<T, -1, 1>& params_r, std::ostream* msgs) const {
     if constexpr (std::is_same<T, double>::value) {

src/test/unit/model/model_base_test.cpp

@@ -23,7 +23,9 @@ struct mock_model : public stan::model::model_base {
                        bool include_gqs) const override {}
   void get_dims(std::vector<std::vector<size_t> >& dimss, bool include_tparams,
                 bool include_gqs) const override {}
-
+  std::string get_transformed_data_json() const override {
+    return std::string("{}");
+  }
   void constrained_param_names(std::vector<std::string>& param_names,
                                bool include_tparams,
                                bool include_gqs) const override {}

src/test/unit/services/util/initialize_test.cpp

@@ -234,6 +234,8 @@ class mock_throwing_model : public stan::model::prob_grad {
       param_names__.push_back(param_name.str());
     }
   }
+  std::string get_transformed_data_json() const { return std::string("{}"); }
+
   template <typename RNG>
   void write_array(RNG& base_rng__, std::vector<double>& params_r__,
                    std::vector<int>& params_i__, std::vector<double>& vars__,

src/test/unit/services/util/mcmc_writer_test.cpp

@@ -129,6 +129,8 @@ class throwing_model : public stan::model::model_base_crtp<throwing_model> {
 
   }  // unconstrained_param_names()
 
+  std::string get_transformed_data_json() const { return std::string("{}"); }
+
   // Begin method overload boilerplate
   template <typename RNG>
   void write_array(RNG& base_rng,