update the test

tests/python/test_ad_atomic_fwd.py

@@ -7,13 +7,13 @@ def test_ad_reduce_fwd():
     N = 16
 
     x = ti.field(dtype=ti.f32, shape=N)
-    loss = ti.field(dtype=ti.f32, shape=N)
+    loss = ti.field(dtype=ti.f32, shape=())
     ti.root.lazy_dual()
 
     @ti.kernel
     def func():
         for i in x:
-            loss[i] += x[i]**2
+            loss[None] += x[i]**2
 
     total_loss = 0
     for i in range(N):
@@ -23,10 +23,9 @@ def func():
     with ti.ad.FwdMode(loss=loss, parameters=x, seed=[1.0 for _ in range(N)]):
         func()
 
-    total_loss_computed = 0
+    assert total_loss == test_utils.approx(loss[None])
+    sum = 0
     for i in range(N):
-        total_loss_computed += loss[i]
+        sum += i * 2
 
-    assert total_loss == test_utils.approx(total_loss_computed)
-    for i in range(N):
-        assert loss.dual[i] == test_utils.approx(i * 2)
+    assert loss.dual[None] == test_utils.approx(sum)