cc: reimplement read_file_to_string without calling TensorFlow (#3176)

LAMMPS is using it

Signed-off-by: Jinzhe Zeng <[email protected]>

source/api_cc/src/common.cc

@@ -4,6 +4,9 @@
 #include <fcntl.h>
 
 #include <cstring>
+#include <fstream>
+#include <sstream>
+#include <string>
 
 #include "AtomMap.h"
 #include "device.h"
@@ -1112,12 +1115,16 @@ template void deepmd::select_map_inv<deepmd::STRINGTYPE>(
 #endif
 
 void deepmd::read_file_to_string(std::string model, std::string& file_content) {
-#ifdef BUILD_TENSORFLOW
-  deepmd::check_status(tensorflow::ReadFileToString(tensorflow::Env::Default(),
-                                                    model, &file_content));
-#else
-  throw deepmd::deepmd_exception("TODO: read_file_to_string only support TF");
-#endif
+  // generated by GitHub Copilot
+  std::ifstream file(model);
+  if (file.is_open()) {
+    std::stringstream buffer;
+    buffer << file.rdbuf();
+    file_content = buffer.str();
+    file.close();
+  } else {
+    throw deepmd::deepmd_exception("Failed to open file: " + model);
+  }
 }
 
 void deepmd::convert_pbtxt_to_pb(std::string fn_pb_txt, std::string fn_pb) {