Fix typo in matrix mult (facebookresearch#3607)

Summary:
Pull Request resolved: facebookresearch#3607

Github issue: facebookresearch#3026

Reviewed By: mengdilin, junjieqi

Differential Revision: D59284732

fbshipit-source-id: a1100c05e7eab40ae90e910364f9ee1db8b1bc55

Author: Michael Norris

faiss/gpu/utils/MatrixMult-inl.cuh

@@ -168,8 +168,8 @@ void runMatrixMult(
     FAISS_ASSERT(c.getSize(0) <= std::numeric_limits<int>::max());
     FAISS_ASSERT(c.getSize(1) <= std::numeric_limits<int>::max());
 
-    FAISS_ASSERT(a.getSize(0) <= std::numeric_limits<int>::max());
-    FAISS_ASSERT(a.getSize(1) <= std::numeric_limits<int>::max());
+    FAISS_ASSERT(b.getSize(0) <= std::numeric_limits<int>::max());
+    FAISS_ASSERT(b.getSize(1) <= std::numeric_limits<int>::max());
 
     FAISS_ASSERT(a.getSize(0) <= std::numeric_limits<int>::max());
     FAISS_ASSERT(a.getSize(1) <= std::numeric_limits<int>::max());