fix bug in pool_dnnlowp_op_avx2.cc (#18141)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18141
VLEN should've been 32
Reviewed By: jianyuh
Differential Revision: D14510780
fbshipit-source-id: ddf12746e1c69677a268432432ddb088cc210084
