Fix onnx import bugs (#4750)
Fix onnx import bugs (#4750)

* Fix onnx import bugs

Fix onnx attributes of string type incorrect handling
Merge symmetric padding of Conv to symmetric form

* Only merge symmetric padding for conv2d

tqchen/tvm,dmlc/tvm,dmlc/tvm,tqchen/tvm,dmlc/tvm,dmlc/tvm,tqchen/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,sxjscience/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,sxjscience/tvm,sxjscience/tvm,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,sxjscience/tvm,sxjscience/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,sxjscience/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,Laurawly/tvm-1,dmlc/tvm,tqchen/tvm,Laurawly/tvm-1