Build fix with certain compilers versions (pytorch#575)

Summary:
Pull Request resolved: pytorch#575

Issue reported in pytorch#573

static constexpr public members of a class should have a definition available if used outside the class.

Reviewed By: shz0116

Differential Revision: D27265594

fbshipit-source-id: b7094f19932e33aafa8489ece2942df71374d250

src/FbgemmSparseDense.cc

@@ -72,6 +72,15 @@ fbgemmDenseToBCSR(int R, int C, const T* inp) {
   return fbgemmDenseToBCSR<T, RB, CB>(R, C, inp, C);
 }
 
+template <typename T, int RB, int CB>
+constexpr int BCSRMatrix<T, RB, CB>::RB;
+
+template <typename T, int RB, int CB>
+constexpr int BCSRMatrix<T, RB, CB>::CB;
+
+template <typename T, int RB, int CB>
+constexpr int BCSRMatrix<T, RB, CB>::COLTILE;
+
 template <typename T, int RB, int CB>
 void BCSRMatrix<T, RB, CB>::pack(const DTYPE* src, size_t ld) {
   rowBPtr.push_back(0);