Fix RNN OP multi-threads predict bug (#41529)

paddle/phi/kernels/cpu/rnn_kernel.cc

@@ -832,11 +832,13 @@ void RnnKernel(const Context& dev_ctx,
                DenseTensor* dropout_state,
                std::vector<DenseTensor*> state,
                DenseTensor* reserve) {
+  if (!is_test) {
     if (dropout_state->IsInitialized()) {
       if (dropout_state->numel() != out->numel()) dropout_state->clear();
     }
     const auto& out_dim = out->dims();
     Full<uint8_t>(dev_ctx, {out_dim.Get(), out_dim.size()}, 1, dropout_state);
+  }
 
   // init the output and allocate the memory
   dev_ctx.template Alloc<T>(out);