Using expanded distance forms in RaftFlatIndex.cu (#3021)

Summary:
This is a minor bug that comes with a perf impact. The classic FAISS `FlatIndex` always uses expanded form of distance computation even though an argument `exactDistances` is provided. `RaftFlatIndex` was using this argument to determine whether the computation should be exhaustive.

This PR includes one additional change to eagerly initialize the `cublas_handle` on the `device_resources` instance when it's created.

Pull Request resolved: #3021

Reviewed By: pemazare

Differential Revision: D48739660

Pulled By: mdouze

fbshipit-source-id: a361334eb243df86c169c69d24bb10fed8876ee9

faiss/gpu/StandardGpuResources.cpp

@@ -432,6 +432,9 @@ raft::device_resources& StandardGpuResourcesImpl::getRaftHandle(int device) {
         // Make sure we are using the stream the user may have already assigned
         // to the current GpuResources
         raftHandles_.emplace(std::make_pair(device, getDefaultStream(device)));
+
+        // Initialize cublas handle
+        raftHandles_[device].get_cublas_handle();
     }
 
     // Otherwise, our base default handle

faiss/gpu/impl/RaftUtils.h

@@ -36,8 +36,7 @@ inline raft::distance::DistanceType faiss_to_raft(
         case MetricType::METRIC_INNER_PRODUCT:
             return raft::distance::DistanceType::InnerProduct;
         case MetricType::METRIC_L2:
-            return exactDistance ? raft::distance::DistanceType::L2Unexpanded
-                                 : raft::distance::DistanceType::L2Expanded;
+            return raft::distance::DistanceType::L2Expanded;
         case MetricType::METRIC_L1:
             return raft::distance::DistanceType::L1;
         case MetricType::METRIC_Linf: