[inductor] fix avg_pool2d accuracy problem in lowering (#101789)
Fixes #100987
In the current `avg_pool2d` lowering of inductor when `count_include_pad`, the mean of each window is calculated by dividing a fixing value, i.e. `kernel_size[0] * kernel_size[1]`. However for ceil mode, the amount of number in a window on the border could be less than `kernel_size[0] * kernel_size[1]`. This PR fixes the issue.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101789
Approved by: https://github.com/jgong5, https://github.com/jansel, https://github.com/EikanWang
