Add VectorTransform read from filename to the C API (facebookresearch#3970)

Summary:
This adds read_VectorTransform to the C API. This is helpful for independently loading a vector transform rather than as an IndexPreTransform.

Pull Request resolved: facebookresearch#3970

Reviewed By: asadoughi

Differential Revision: D65192203

Pulled By: junjieqi

fbshipit-source-id: 949ae875924b9f3558d7a9f43c4f2aa8ae705f02

Author: makosten

c_api/index_io_c.cpp

@@ -14,6 +14,7 @@
 
 using faiss::Index;
 using faiss::IndexBinary;
+using faiss::VectorTransform;
 
 int faiss_write_index(const FaissIndex* idx, FILE* f) {
     try {
@@ -83,3 +84,13 @@ int faiss_read_index_binary_fname(
     }
     CATCH_AND_HANDLE
 }
+
+int faiss_read_VectorTransform_fname(
+        const char* fname,
+        FaissVectorTransform** p_out) {
+    try {
+        auto out = faiss::read_VectorTransform(fname);
+        *p_out = reinterpret_cast<FaissVectorTransform*>(out);
+    }
+    CATCH_AND_HANDLE
+}

c_api/index_io_c.h

@@ -14,6 +14,7 @@
 #include <stdio.h>
 #include "IndexBinary_c.h"
 #include "Index_c.h"
+#include "VectorTransform_c.h"
 #include "faiss_c.h"
 
 #ifdef __cplusplus
@@ -71,6 +72,13 @@ int faiss_read_index_binary_fname(
         const char* fname,
         int io_flags,
         FaissIndexBinary** p_out);
+
+/** Read vector transform from a file.
+ * This is equivalent to `faiss:read_VectorTransform` when a file path is given.
+ */
+int faiss_read_VectorTransform_fname(
+        const char* fname,
+        FaissVectorTransform** p_out);
 #ifdef __cplusplus
 }
 #endif