Fix deprecated use of 0/NULL in gloo/cuda_allreduce_bcube.h + 1

Summary:
`nullptr` is typesafe. `0` and `NULL` are not. In the future, only `nullptr` will be allowed.

This diff helps us embrace the future _now_ in service of enabling `-Wzero-as-null-pointer-constant`.

Reviewed By: palmje

Differential Revision: D56650319

fbshipit-source-id: 3d9dbb5972173636c9512677f7d78674f61f1df4

Author: r-barnes

gloo/cuda_allreduce_bcube.h

@@ -251,13 +251,13 @@ class CudaAllreduceBcube : public Algorithm {
   void init(
       typename std::enable_if<
           std::is_same<U, CudaHostWorkspace<T>>::value,
-          typename U::Pointer>::type* = 0);
+          typename U::Pointer>::type* = nullptr);
 
   template <typename U = W>
   void init(
       typename std::enable_if<
           std::is_same<U, CudaDeviceWorkspace<T>>::value,
-          typename U::Pointer>::type* = 0);
+          typename U::Pointer>::type* = nullptr);
 };
 
 namespace cuda {