Fix deprecated use of 0/NULL in gloo/cuda_allreduce_ring.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: D56650277

fbshipit-source-id: a9be03583ac815de0b2b2d240adad4d3aa42d47e

Author: r-barnes

gloo/cuda_allreduce_ring.h

@@ -32,12 +32,12 @@ class CudaAllreduceRing : 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);
 
   std::vector<CudaDevicePointer<T> > devicePtrs_;
   std::vector<CudaStream> streams_;