chore(gpu): add comments inside host_integer_sum_ciphertexts_vec_kb f…

…unction

backends/tfhe-cuda-backend/cuda/src/integer/multiplication.cuh

@@ -328,6 +328,9 @@ __host__ void host_integer_sum_ciphertexts_vec_kb(
     cuda_memcpy_async_to_gpu(d_smart_copy_out, h_smart_copy_out, copy_size,
                              streams[0], gpu_indexes[0]);
 
+    // inside d_smart_copy_in there are only -1 values
+    // it's fine to call smart_copy with same pointer
+    // as source and destination
     smart_copy<<<sm_copy_count, 1024, 0, streams[0]>>>(
         new_blocks, new_blocks, d_smart_copy_out, d_smart_copy_in,
         big_lwe_size);