Fix incorrect size of grid dimension in index_select (#54660)

paddle/phi/kernels/gpu/index_select_grad_kernel.cu

@@ -87,7 +87,7 @@ void IndexSelectGradKernel(const Context& ctx,
   auto stream = ctx.stream();
 
   unsigned int block_dim = PADDLE_CUDA_NUM_THREADS;
-  dim3 grid_dim = dim3((numel + block_dim - 1) / block_dim);
+  dim3 grid_dim = dim3((out_nums + block_dim - 1) / block_dim);
   phi::backends::gpu::LimitGridDim(ctx, &grid_dim);
 
   phi::funcs::SetConstant<phi::GPUContext, T> index_select_grad_init;