modify gather_op with test

paddle/operators/gather_op.cc

@@ -27,6 +27,9 @@ class GatherOp : public framework::OperatorWithKernel {
                             "Inputs of GatherOp must all be set");
     int batch_size = ctx.Input<Tensor>(1)->dims()[0];
     PADDLE_ENFORCE(batch_size > 0);
+    paddle::framework::DDim output_dims(ctx.Input<Tensor>(0)->dims());
+    output_dims[0] = batch_size;
+    ctx.Output<Tensor>(0)->Resize(output_dims);
   }
 };
 
@@ -48,8 +51,8 @@ Y = X[Index]
 class GatherGradOp : public framework::OperatorWithKernel {
  protected:
   void InferShape(const framework::InferShapeContext &ctx) const override {
-    // ctx.Output<Tensor>("X" + framework::kGradVarSuffix)
-    //    ->Resize(ctx.Input<Tensor>("X")->dims());
+    ctx.Output<Tensor>("X" + framework::kGradVarSuffix)
+        ->Resize(ctx.Input<Tensor>("X")->dims());
   }
 };