Created by: lidanqing-intel

This PR improve performance of density_prior_box_op ~3 times faster

• test machine: SKX 8180.
• test script: ./test_analyzer_resnet50 --infer_model=models/detect --gtest_filter=Analyzer_resnet50.profile_mkldnn --paddle_num_threads=4 --repeat=1 --batch_size=1 --profile

test=develop