sync with FB version 2018-02-23 (#347)

- support on-disk IVF

MetaIndexes.cpp

@@ -422,17 +422,6 @@ void IndexShards::add (idx_t n, const float *x)
     add_with_ids (n, x, nullptr);
 }
 
- /**
-  * Cases (successive_ids, xids):
-  * - true, non-NULL       ERROR: it makes no sense to pass in ids and
-  *                        request them to be shifted
-  * - true, NULL           OK, but should be called only once (calls add()
-  *                        on sub-indexes).
-  * - false, non-NULL      OK: will call add_with_ids with passed in xids
-  *                        distributed evenly over shards
-  * - false, NULL          OK: will call add_with_ids on each sub-index,
-  *                        starting at ntotal
-  */
 
 void IndexShards::add_with_ids (idx_t n, const float * x, const long *xids)
 {