loosly check in the InferShape of cross_entropy_op. (#15863)

* loosly check in cross_entropy_op when soft_label is True
* Add Runtime assertion in backward infer_shape check.
* Skip InferShape check when un-know the input dimensions

paddle/fluid/operators/cross_entropy_op.cc

@@ -32,14 +32,23 @@ class CrossEntropyOp : public framework::OperatorWithKernel {
     int rank = x_dims.size();
     PADDLE_ENFORCE_EQ(rank, label_dims.size(),
                       "Input(X) and Input(Label) shall have the same rank.");
+    bool check = true;
+    if ((!ctx->IsRuntime()) && (framework::product(x_dims) <= 0 ||
+                                framework::product(label_dims) <= 0)) {
+      check = false;
+    }
+    if (check) {
       PADDLE_ENFORCE_EQ(framework::slice_ddim(x_dims, 0, rank - 1),
                         framework::slice_ddim(label_dims, 0, rank - 1),
                         "Input(X) and Input(Label) shall have the same shape "
                         "except the last dimension.");
+    }
     if (ctx->Attrs().Get<bool>("soft_label")) {
+      if (check) {
         PADDLE_ENFORCE_EQ(x_dims[rank - 1], label_dims[rank - 1],
                           "If Attr(soft_label) == true, the last dimension of "
                           "Input(X) and Input(Label) should be equal.");
+      }
     } else {
       PADDLE_ENFORCE_EQ(label_dims[rank - 1], 1UL,
                         "If Attr(softLabel) == false, the last dimension of "
@@ -82,6 +91,14 @@ class CrossEntropyGradientOp : public framework::OperatorWithKernel {
                       "Input(Y@Grad) and Input(X) should have the same rank.");
     PADDLE_ENFORCE_EQ(label_dims.size(), rank,
                       "Input(Label) and Input(X) should have the same rank.");
+
+    bool check = true;
+    if ((!ctx->IsRuntime()) && (framework::product(x_dims) <= 0 ||
+                                framework::product(label_dims) <= 0)) {
+      check = false;
+    }
+
+    if (check) {
       PADDLE_ENFORCE_EQ(framework::slice_ddim(x_dims, 0, rank - 1),
                         framework::slice_ddim(label_dims, 0, rank - 1),
                         "The Input(X) and Input(Label) should have the same "
@@ -90,12 +107,16 @@ class CrossEntropyGradientOp : public framework::OperatorWithKernel {
                         framework::slice_ddim(dy_dims, 0, rank - 1),
                         "The Input(X) and Input(Y@Grad) should have the same "
                         "shape except the last dimension.");
+    }
     PADDLE_ENFORCE_EQ(dy_dims[rank - 1], 1,
                       "The last dimension of Input(Y@Grad) should be 1.");
     if (ctx->Attrs().Get<bool>("soft_label")) {
-      PADDLE_ENFORCE_EQ(x_dims[rank - 1], label_dims[rank - 1],
+      if (check) {
+        PADDLE_ENFORCE_EQ(
+            x_dims[rank - 1], label_dims[rank - 1],
             "When Attr(soft_label) == true, the last dimension of "
             "Input(X) and Input(Label) should be equal.");
+      }
     } else {
       PADDLE_ENFORCE_EQ(label_dims[rank - 1], 1,
                         "When Attr(soft_label) == false, the last dimension of "