Update model_spec.py

Author: BBC-Esq

python/ctranslate2/specs/model_spec.py

@@ -749,15 +749,15 @@ def num_bytes(self) -> int:
     def to_bytes(self) -> bytes:
         max_size = 2**31 - 1
         num_bytes = self.num_bytes()
-        output = b""
+        chunks = []
         offset = 0
         while num_bytes > 0:
             chunk_size = max_size if num_bytes > max_size else num_bytes
             chunk = ctypes.string_at(self.tensor.data_ptr() + offset, chunk_size)
-            output += chunk
+            chunks.append(chunk)  # Collect chunks in a list
             offset += chunk_size
             num_bytes -= chunk_size
-        return output
+        return b"".join(chunks)
 
     def _to(self, dtype: str) -> Variable:
         dtype = getattr(torch, dtype)