[aten] remove warning on change of sign (#34016)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34016
Remove warning
```
caffe2/aten/src/ATen/native/cuda/Reduce.cuh(654): warning: integer conversion resulted in a change of sign
```
When acc_ptr_ != nullptr , numerator_ and denominator_ must have been initialized.
Other minor changes:
* Make member variables of AccumulationBuffer private
* size_factor_ is used nowhere.
Test Plan: CI
Reviewed By: dskhudia
Differential Revision: D20181169
fbshipit-source-id: e4d023f7fa0692e62be21cfbd971cad8dfb69ea4
