Speed up sr accumulation (#42658)

* Support Gradient Accumulation for sr

* add ut

* change ut to fit small vector

* speed up accumulation

paddle/fluid/eager/accumulation/accumulation_node.cc

@@ -34,22 +34,9 @@ static void CopyOrAddTensor(paddle::experimental::Tensor* tensor,
     *tensor = t;
   } else {
     // Accumulation
-    PADDLE_ENFORCE_EQ(t.initialized(), true,
-                      paddle::platform::errors::Fatal(
-                          "We can only accumulate initialized tensor, but we "
-                          "got tensor: %s is empty please check you network "
-                          "and make sure it creates grads.",
-                          t.name()));
-    PADDLE_ENFORCE_NOT_NULL(
-        tensor, paddle::platform::errors::Fatal(
-                    "We can only accumulate initialized tensor to non-nullptr "
-                    "tensor but we got nullptr please check you network "
-                    "and make sure it creates grads."));
-
-    if (t.is_dense_tensor()) {
-      if (tensor->is_dense_tensor()) {
+    if (LIKELY(t.is_dense_tensor())) {
+      if (LIKELY(tensor->is_dense_tensor())) {
         paddle::imperative::TensorAdd<paddle::experimental::Tensor>(t, tensor);
-
       } else {
         // TODO(jiabin): Support Other TensorBase later
         // TODO(zhanlve): Replace SelectedRowsAddTensor with