Fix the back_end value if TREXIO_AUTO is used

src/templates_front/templator_front.org

@@ -742,7 +742,7 @@ struct trexio_s {
 
    #+begin_src python :tangle basic_python.py
 class File:
-    """TREXIO File object.
+    """trexio.File class.
 
     General information about the TREXIO file.
 
@@ -775,7 +775,6 @@ class File:
         """TREXIO File class constructor."""
         self.filename = filename
         self.mode = mode
-        self.back_end = back_end
         self.state = 0
 
         self.isOpen = False
@@ -789,6 +788,7 @@ class File:
             self.isOpen = None
             self.exists = None
 
+        self.back_end = self.pytrexio_s.back_end
         self.info = info