minor refactor to avoid else block in IVFPQ reconstruct_from_offset. (facebookresearch#3753)

Summary:
Pull Request resolved: facebookresearch#3753

minor refactor to avoid else block in IVFPQ reconstruct_from_offset. No change in logic.

Reviewed By: asadoughi

Differential Revision: D61255339

fbshipit-source-id: e0a8ac10570391eaf7ed3b35796af8b38d40a23c

Author: pankajsingh88

faiss/IndexIVFPQ.cpp

@@ -318,16 +318,14 @@ void IndexIVFPQ::reconstruct_from_offset(
         float* recons) const {
     const uint8_t* code = invlists->get_single_code(list_no, offset);
 
+    pq.decode(code, recons);
     if (by_residual) {
         std::vector<float> centroid(d);
         quantizer->reconstruct(list_no, centroid.data());
 
-        pq.decode(code, recons);
         for (int i = 0; i < d; ++i) {
             recons[i] += centroid[i];
         }
-    } else {
-        pq.decode(code, recons);
     }
 }