Fix deprecated use of 0/NULL in gloo/cuda_allreduce_halving_doubling.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: D56650289

fbshipit-source-id: 0a320e181876b91bfea27a8400f0a1eb4b827ed9

Author: r-barnes

gloo/cuda_allreduce_halving_doubling.h

@@ -42,17 +42,17 @@ class CudaAllreduceHalvingDoubling : public Algorithm {
   template <typename U = W>
   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);
 
   template <typename U = W>
   void initReductionsAndBroadcasts(
       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 initReductionsAndBroadcasts(