Do not use os.path.join for variable paths. (#21680)

Variable paths do not represent a filesystem path and should not be OS dependent, they should always use "/" as a separator.

This reverts #21343 just for variables.

Author: hertschuh

keras/src/backend/common/variables.py

@@ -1,5 +1,3 @@
-import os.path
-
 import numpy as np
 
 from keras.src import backend
@@ -144,7 +142,7 @@ def __init__(
         self._name = name
         parent_path = current_path()
         if parent_path:
-            self._path = os.path.join(current_path(), name)
+            self._path = f"{parent_path}/{name}"
         else:
             self._path = name
         self._shape = None

keras/src/backend/common/variables_test.py

@@ -328,6 +328,10 @@ def test_variable_path_creation(self):
         v = backend.Variable(initializer=np.ones((2, 2)), name="test_var")
         self.assertEqual(v.path, "test_var")
 
+        with backend.name_scope("test_scope"):
+            v = backend.Variable(initializer=np.ones((2, 2)), name="test_var")
+            self.assertEqual(v.path, "test_scope/test_var")
+
     def test_overwrite_with_gradient_setter(self):
         v = backend.Variable(
             initializer=initializers.RandomNormal(),