fix index_select kernel configuration error where input numel is 0 (#41383)

paddle/phi/kernels/gpu/index_select_grad_kernel.cu

@@ -85,6 +85,9 @@ void IndexSelectGradKernel(const Context& ctx,
                         phi::DataType::INT64));
 
   int64_t numel = x_grad->numel();
+  if (numel == 0) {
+    return;
+  }
   int64_t index_nums = index.numel();
   int64_t out_nums = out_grad.numel();
 

paddle/phi/kernels/gpu/index_select_kernel.cu

@@ -72,6 +72,9 @@ void IndexSelectKernel(const Context& ctx,
   T* out_data = ctx.template Alloc<T>(output);
 
   int64_t numel = output->numel();
+  if (numel == 0) {
+    return;
+  }
   auto stream = ctx.stream();
 
   unsigned int block_dim = PADDLE_CUDA_NUM_THREADS;