[v1.x] Fix gcc 10 build

[mseth10]

Description

Cherry-pick 78e31d6 and 9bfe311 from master to v1.x branch. Fixes gcc-10 build which gives the following error without this fix:

/usr/bin/ld: libmxnet.a(elemwise_binary_broadcast_op_basic.cc.o): in function `void mxnet::op::BinaryBroadcastComputeDenseEx<mshadow::cpu, mxnet::op::mshadow_op::minus>(nnvm::NodeAttrs const&, mxnet::OpContext const&, std::vector<mxnet::NDArray, std::allocator<mxnet::NDArray> > const&, std::vector<mxnet::OpReqType, std::allocator<mxnet::OpReqType> > const&, std::vector<mxnet::NDArray, std::allocator<mxnet::NDArray> > const&)':
elemwise_binary_broadcast_op_basic.cc:(.text._ZN5mxnet2op29BinaryBroadcastComputeDenseExIN7mshadow3cpuENS0_10mshadow_op5minusEEEvRKN4nnvm9NodeAttrsERKNS_9OpContextERKSt6vectorINS_7NDArrayESaISE_EERKSD_INS_9OpReqTypeESaISJ_EESI_[_ZN5mxnet2op29BinaryBroadcastComputeDenseExIN7mshadow3cpuENS0_10mshadow_op5minusEEEvRKN4nnvm9NodeAttrsERKNS_9OpContextERKSt6vectorINS_7NDArrayESaISE_EERKSD_INS_9OpReqTypeESaISJ_EESI_]+0x1aa4): undefined reference to `void mxnet::op::ElemwiseBinaryOp::DnsCsrDnsOp<mxnet::op::mshadow_op::minus>(mshadow::Stream<mshadow::cpu>*, nnvm::NodeAttrs const&, mxnet::OpContext const&, mxnet::NDArray const&, mxnet::NDArray const&, mxnet::OpReqType, mxnet::NDArray const&, bool)'

[mxnet-bot]

Hey @mseth10 , Thanks for submitting the PR
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[mseth10]

Thanks @josephevans for the suggestion. Can you please review?

[josephevans]

LGTM, thanks!