Enable OMP multithreading in lookup_table_v2 (#45249)

* Add omp parallel for directives

* Revert "Add omp parallel for directives"

This reverts commit f4e4f8ddb12454018d9c1e49c074af2543659de6.

* Add #pragma omp parallel for to correct file

* Add check for _OPENMP definition

* Disable omp on gpu

* Trigger CI

* Readd check for _OPENMP definition

* Change macro disabling changes on GPU

* Improve macro readability

paddle/phi/kernels/cpu/embedding_kernel.cc

@@ -48,6 +48,10 @@ struct EmbeddingCPUFunctor {
     dev_ctx_.template Alloc<T>(out_);
     auto* output = out_->data<T>();
 
+#if defined(_OPENMP) && !defined(PADDLE_WITH_CUDA)
+#pragma omp parallel for
+#endif
+
     for (int64_t i = 0; i < ids_numel; ++i) {
       if (padding_idx_ != kNoPadding && ids[i] == padding_idx_) {
         memset(output + i * row_width, 0, row_width * sizeof(T));