libspconv.so has memory leakage

[Junx0924]

Hi, thanks very much for the excellent work of libspconv.

When we analyzed the memory usage of spconv, it seems that some code in the so file is responsible for the memory leak.

Could you help to fix it? Thanks a lot.

==19934== 23,813 (504 direct, 23,309 indirect) bytes in 21 blocks are definitely lost in loss record 1,832 of 1,998 ==19934== at 0x4C33823: operator new(unsigned long) (vg_replace_malloc.c:472) ==19934== by 0x29AD5019: spconv::SparseConvNode::SparseConvNode(std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&, spconv::MiniTensor*, onnx::NodeProto const&, onnx::TensorProto const&, onnx::TensorProto const&) (in /opt/cpp_libs/libspconv/lib/x86_64/libspconv.so) ==19934== by 0x29AD6BA1: spconv::load_engine_from_onnx(std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&, spconv::Precision) (in /opt/cpp_libs/libspconv/lib/x86_64/libspconv.so)

[hopef]

I will check this issue. Thanks a lot.