fix typo: forward->backward (#872)

Co-authored-by: holly1238 <77758406+holly1238@users.noreply.github.com>

Author: crcrpar

advanced_source/cpp_extension.rst

@@ -1101,7 +1101,7 @@ on it:
     const int threads = 1024;
     const dim3 blocks((state_size + threads - 1) / threads, batch_size);
 
-    AT_DISPATCH_FLOATING_TYPES(X.type(), "lltm_forward_cuda", ([&] {
+    AT_DISPATCH_FLOATING_TYPES(X.type(), "lltm_backward_cuda", ([&] {
       lltm_cuda_backward_kernel<scalar_t><<<blocks, threads>>>(
           d_old_cell.packed_accessor32<scalar_t,2,torch::RestrictPtrTraits>(),
           d_gates.packed_accessor32<scalar_t,3,torch::RestrictPtrTraits>(),