XNN_FLAG_KEEP_DIM not backwards compatible

[mcr229]

Commit:
23f386b

Introduces the XNN_FLAG_KEEP_DIM flag which makes global_sum_pooling, global_average_pooling, and static mean no longer backwards compatible. That is calls previously to

xnn_define_global_average_pooling will reduce dims, when previous to this commit they would not. I would like to suggest that this flag be changed to XNN_FLAG_REDUCE_DIMS. This way previous calls to global_sum_pooling, global_average_pooling, and static mean is not changed(keep dim is on by deafult) , and instead we opt-in to reducing dims with this flag.

PR for change is here:
#6089

[mcr229]

hi @alankelly, I noticed that these changes were rolled back. Is that the intention for the future going forward? Just want to know now to understand whether or not the commit with flag_reduce_dims is stable or not.