Add fp16 training for ResNeXt101

[huangxu96]

This PR adds fp16 training for ResNeXt101.

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8 out of 28 committers have signed the CLA.

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

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