Unify the code path between local and distributed training. (#9433)

This removes the need for a local histogram space during distributed training, which cuts the cache size by half.

src/common/hist_util.h

@@ -453,6 +453,7 @@ class HistCollection {
       data_[0].resize(new_size);
     }
   }
+  [[nodiscard]] bool IsContiguous() const { return contiguous_allocation_; }
 
  private:
   /*! \brief number of all bins over all features */