Add fp16 training for ResNeXt101

[huangxu96]

This PR adds fp16 training for ResNeXt101.

[CLAassistant]

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28 out of 29 committers have signed the CLA.

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

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