Migrate IVF-Flat from RAFT (#94)

Authors:
  - Divye Gala (https://github.com/divyegala)
  - Micka (https://github.com/lowener)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #94

cpp/CMakeLists.txt

@@ -379,18 +379,16 @@ add_library(
   src/neighbors/detail/refine_host_int8_t_float.cpp
   src/neighbors/detail/refine_host_uint8_t_float.cpp
   src/neighbors/ivf_flat_index.cpp
-  src/neighbors/ivf_flat/ivf_flat_build_float_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_build_int8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_build_uint8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_extend_float_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_extend_int8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_extend_uint8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_search_float_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_search_int8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_search_uint8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_serialize_float_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_serialize_int8_t_int64_t.cpp
-  src/neighbors/ivf_flat/ivf_flat_serialize_uint8_t_int64_t.cpp
+  src/neighbors/ivf_flat/ivf_flat_build_extend_float_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_build_extend_int8_t_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_build_extend_uint8_t_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_helpers.cu
+  src/neighbors/ivf_flat/ivf_flat_search_float_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_search_int8_t_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_search_uint8_t_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_serialize_float_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_serialize_int8_t_int64_t.cu
+  src/neighbors/ivf_flat/ivf_flat_serialize_uint8_t_int64_t.cu
   src/neighbors/ivf_pq_index.cpp
   src/neighbors/ivf_pq/ivf_pq_build_common.cu
   src/neighbors/ivf_pq/ivf_pq_serialize.cu