Avoid fallback for avg_pool - (#6409)
By supporting divisor overrides and ceil_mode + count_include_pad property.
When count_include_pad is True, the number of paddings also appear in the denominator. But, if ceil_mode is also true, then, when we round, we can introduce extra padding. ** these paddings are NOT counted in the denominator** Therefore, when ceil_mode is true, we need to manually pad to distinguish padding that should count for denominator and those that shouldnt (i.e. those introduced by ceil_mode)
