move TrainableIndex interface to the right place (#87)

Author: Thejas-bhat

index.go

@@ -65,6 +65,11 @@ type IndexFileCopyable interface {
 	CopyFile(file string, d IndexDirectory) error // source index
 }
 
+type TrainableIndex interface {
+	Index
+	Train(*Batch) error
+}
+
 // EventIndex is an optional interface for exposing the support for firing event
 // callbacks for various events in the index.
 type EventIndex interface {

vector_index.go

@@ -85,8 +85,3 @@ func (vd *VectorDoc) Reset() *VectorDoc {
 	vd.ID = id[:0]
 	return vd
 }
-
-type TrainableIndex interface {
-	Index
-	Train(*Batch) error
-}