Add fp16 training for ResNeXt101

[huangxu96]

This PR adds fp16 training for ResNeXt101.

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28 out of 29 committers have signed the CLA.

:white_check_mark: FlyingQianMM
:white_check_mark: frankwhzhang
:white_check_mark: huangjun12
:white_check_mark: phlrain
:white_check_mark: jczaja
:white_check_mark: ceci3
:white_check_mark: chenwhql
:white_check_mark: LDOUBLEV
:white_check_mark: Xreki
:white_check_mark: ChinaLiuHao
:white_check_mark: anpark
:white_check_mark: littletomatodonkey
:white_check_mark: hysunflower
:white_check_mark: qingqing01
:white_check_mark: arlesniak
:white_check_mark: chajchaj
:white_check_mark: xbsu
:white_check_mark: shuluoshu
:white_check_mark: DrRyanHuang
:white_check_mark: a2824256
:white_check_mark: LiuChiachi
:white_check_mark: wanghuancoder
:white_check_mark: LiuChaoXD
:white_check_mark: zhiqiu
:white_check_mark: shippingwang
:white_check_mark: zhangting2020
:white_check_mark: luotao1
:white_check_mark: huangxu96
:x: tianxin

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