Wrap model attributes in smart ptrs to avoid memory leaks

Remove memory leaks from #57 and #61 by using shared ptrs on:
	model::graph
	model::session
And unique ptrs on vars from model constructor:
	session_options
	run_options
	meta_graph

include/cppflow/model.h

@@ -25,29 +25,40 @@ namespace cppflow {
         std::vector<tensor> operator()(std::vector<std::tuple<std::string, tensor>> inputs, std::vector<std::string> outputs);
         tensor operator()(const tensor& input);
 
+        ~model() = default;
+        model(const model &model) = default;
+        model(model &&model) = default;
+        model &operator=(const model &other) = default;
+        model &operator=(model &&other) = default;
+
     private:
 
-        TF_Graph* graph;
-        TF_Session* session;
+        std::shared_ptr<TF_Graph> graph;
+        std::shared_ptr<TF_Session> session;
     };
 }
 
+
 namespace cppflow {
 
     model::model(const std::string &filename) {
-        this->graph = TF_NewGraph();
+        this->graph = {TF_NewGraph(), TF_DeleteGraph};
 
         // Create the session.
-        TF_SessionOptions* session_options = TF_NewSessionOptions();
-        TF_Buffer* run_options = TF_NewBufferFromString("", 0);
-        TF_Buffer* meta_graph = TF_NewBuffer();
+        std::unique_ptr<TF_SessionOptions, decltype(&TF_DeleteSessionOptions)> session_options = {TF_NewSessionOptions(), TF_DeleteSessionOptions};
+        std::unique_ptr<TF_Buffer, decltype(&TF_DeleteBuffer)> run_options = {TF_NewBufferFromString("", 0), TF_DeleteBuffer};
+        std::unique_ptr<TF_Buffer, decltype(&TF_DeleteBuffer)> meta_graph = {TF_NewBuffer(), TF_DeleteBuffer};
+
+        auto session_deleter = [](TF_Session* sess) {
+            TF_DeleteSession(sess, context::get_status());
+            status_check(context::get_status());
+        };
 
         int tag_len = 1;
         const char* tag = "serve";
-        this->session = TF_LoadSessionFromSavedModel(session_options, run_options, filename.c_str(), &tag, tag_len, graph, meta_graph, context::get_status());
-        TF_DeleteSessionOptions(session_options);
-        TF_DeleteBuffer(run_options);
-        //TF_DeleteBuffer(meta_graph);
+        this->session = {TF_LoadSessionFromSavedModel(session_options.get(), run_options.get(), filename.c_str(),
+                                &tag, tag_len, this->graph.get(), meta_graph.get(), context::get_status()),
+                         session_deleter};
 
         status_check(context::get_status());
     }
@@ -58,7 +69,7 @@ namespace cppflow {
         TF_Operation* oper;
 
         // Iterate through the operations of a graph
-        while ((oper = TF_GraphNextOperation(this->graph, &pos)) != nullptr) {
+        while ((oper = TF_GraphNextOperation(this->graph.get(), &pos)) != nullptr) {
             result.emplace_back(TF_OperationName(oper));
         }
         return result;
@@ -77,7 +88,7 @@ namespace cppflow {
 
             // Operations
             const auto[op_name, op_idx] = parse_name(std::get<0>(inputs[i]));
-            inp_ops[i].oper = TF_GraphOperationByName(this->graph, op_name.c_str());
+            inp_ops[i].oper = TF_GraphOperationByName(this->graph.get(), op_name.c_str());
             inp_ops[i].index = op_idx;
 
             if (!inp_ops[i].oper)
@@ -94,15 +105,15 @@ namespace cppflow {
         for (int i=0; i<outputs.size(); i++) {
 
             const auto[op_name, op_idx] = parse_name(outputs[i]);
-            out_ops[i].oper = TF_GraphOperationByName(this->graph, op_name.c_str());
+            out_ops[i].oper = TF_GraphOperationByName(this->graph.get(), op_name.c_str());
             out_ops[i].index = op_idx;
 
             if (!out_ops[i].oper)
                 throw std::runtime_error("No operation named \"" + op_name + "\" exists");
 
         }
 
-        TF_SessionRun(this->session, NULL,
+        TF_SessionRun(this->session.get(), NULL,
                 inp_ops.data(), inp_val.data(), inputs.size(),
                 out_ops.data(), out_val.get(), outputs.size(),
                 NULL, 0,NULL , context::get_status());