[example] Fix jacobian example (#6849)

Issue: #6733 

The the grad field of variable at the left side of the global store will
be cleared after #6579. Change to atomic add sort the issue.

python/taichi/examples/autodiff/jacobian.py

@@ -15,7 +15,7 @@
 def func():
     for j in range(N_loss):
         for i in range(N_param):
-            losses[j] = ti.sin(j / 16 * np.pi) * ti.sin(
+            losses[j] += ti.sin(j / 16 * np.pi) * ti.sin(
                 i / 16 * np.pi) * params[i]