[XPU] fix layer_norm_grad bug when bias_grad and scale_grad are nullptr (#54669)

paddle/phi/kernels/xpu/layer_norm_grad_kernel.cc

@@ -63,11 +63,15 @@ void LayerNormGradKernel(const Context& ctx,
     PADDLE_ENFORCE_XDNN_SUCCESS(r, "cast");
     scale_data_fp32 = scale_data_temp;
     need_cast_scale = true;
-    scale_grad_data_fp32 = RAII_GUARD.alloc_l3_or_gm<float>(scale_ptr->numel());
+    scale_grad_data_fp32 =
+        scale_grad == nullptr
+            ? nullptr
+            : RAII_GUARD.alloc_l3_or_gm<float>(scale_ptr->numel());
   } else {
     // no need to cast
     scale_data_fp32 = scale_ptr->data<float>();
-    scale_grad_data_fp32 = ctx.template Alloc<float>(scale_grad);
+    scale_grad_data_fp32 =
+        scale_grad == nullptr ? nullptr : ctx.template Alloc<float>(scale_grad);
   }
 
   // bias
@@ -79,10 +83,14 @@ void LayerNormGradKernel(const Context& ctx,
   } else if (bias_ptr->dtype() ==
              phi::CppTypeToDataType<phi::dtype::float16>::Type()) {
     need_cast_bias = true;
-    bias_grad_data_fp32 = RAII_GUARD.alloc_l3_or_gm<float>(bias_ptr->numel());
+    bias_grad_data_fp32 =
+        bias_grad == nullptr
+            ? nullptr
+            : RAII_GUARD.alloc_l3_or_gm<float>(bias_ptr->numel());
   } else {
     // no need to cast
-    bias_grad_data_fp32 = ctx.template Alloc<float>(bias_grad);
+    bias_grad_data_fp32 =
+        bias_grad == nullptr ? nullptr : ctx.template Alloc<float>(bias_grad);
   }
 
   auto* x_grad_data =