Fix invalid argument of clip when running ssd inference

[wuxun-zhang]

@pengzhao-intel @xinyu-intel @zhreshold

This PR is to make the default value of clip in NormalizedBoxCenterDecoder() aligned with the value in MXNet (defined in bounding_box-inl.h#L939, introduced by PR#16215) .

Before, when running ssd inference, there will be an error, like below:

>>>  python eval_ssd.py --network=mobilenet1.0 --data-shape=512 --batch-size=1
......
mxnet.base.MXNetError: Invalid Parameter format for clip expect float but value='None', in operator _contrib_box_decode(name="", format="center", std2="0.2", clip="None", std3="0.2", std1="0.1", std0="0.1")

[mli]

Job PR-1026-1 is done.
Docs are uploaded to http://gluon-vision-staging.s3-website-us-west-2.amazonaws.com/PR-1026/1/index.html
Code coverage of this PR: vs. Master: