fix(mge): fix backward graph optimization

GitOrigin-RevId: 28fd00ac548e7cd663a87ce74070862e9a24b55f

imperative/python/test/unit/core/test_autodiff.py

@@ -19,7 +19,7 @@ import megengine.functional as F
 from megengine.core._imperative_rt import CompNode, TensorAttr, imperative
 from megengine.core._imperative_rt.core2 import TensorWeakRef, apply, sync
 from megengine.core.autodiff.grad import Grad
-from megengine.core.ops.builtin import Elemwise
+from megengine.core.ops.builtin import Elemwise, Identity
 from megengine.distributed.helper import get_device_count_by_fork
 from megengine.functional.distributed import remote_recv, remote_send
 
@@ -193,6 +193,20 @@ def test_grad_inplace():
     np.testing.assert_almost_equal(x.grad.numpy(), 4 * x_np ** 3, decimal=6)
 
 
+def test_identity():
+    x_np = np.random.rand(10).astype("float32")
+    x = mge.Tensor(x_np)
+    dy_np = np.random.rand(*x.shape).astype("float32")
+    dy = mge.Tensor(dy_np)
+
+    grad = Grad().wrt(x, callback=save_to(x))
+
+    (y,) = apply(Identity(), x)
+
+    grad(y, dy)
+    np.testing.assert_array_equal(x.grad.numpy(), dy_np)
+
+
 def test_elemwise_add():
     x_np = np.random.rand(10).astype("float32")
     y_np = np.random.rand(10, 10).astype("float32")

imperative/src/impl/backward_graph_opt.cpp

@@ -58,7 +58,7 @@ OptimizedBackwardGraphResult::OptimizedBackwardGraphResult(const BackwardGraphRe
     // should be marked as always appears in backward
     for (size_t i = 0, j = 0; i < mask.size(); ++i) {
         if (!mask[i]) continue;
-        if (i > input_size + output_size) {
+        if (i >= input_size + output_size) {
             vinfo[graph.inputs[j]].appears_in_backward = true;
         }
         ++j;