Reuse prepacked pmat for PackBMatrix if smat is nullptr (#4665)

Summary:
X-link: facebookresearch/FBGEMM#1700

This patch solves the problem addressed in #427 and #542 in a more non-intrusive way: the PackBMatrix constructor now skips the actual packing and allows reuse of `pmat` if `smat == nullptr`. This feature is crucial for offline packing and caching `pmat` for better load speed.

Doing so eliminates the need for a separate constructor and ensures the pack parameters are identical.

Pull Request resolved: #4665

Reviewed By: cthi

Differential Revision: D79940804

Pulled By: q10

fbshipit-source-id: 3fb35c18a87cc2d8ec89fdabcee32cfbdaacc4b2

Author: omegacoleman

src/PackBMatrix.cc

@@ -248,6 +248,8 @@ PackBMatrix<T, accT>::PackBMatrix(
         64,
         BaseType::numGroups() * BaseType::blockRows() * BaseType::brow_ *
             BaseType::blockCols() * BaseType::bcol_ * sizeof(T)));
+  } else if (!smat) {
+    return;
   }
   pack(block, params);
 }