Simplify the flow control in div_kernel_cuda. (#22555)
Summary:
Some duplicated code is removed. It also becomes clear that there is only one special case `div_kernel_cuda` is handling.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22555
Differential Revision: D16152091
Pulled By: zou3519
fbshipit-source-id: bb875370077c1f84efe4b766b3e1acc461e73e6c
