Need more explicit instruction on installing the Protobuf and ONNX optimizer.

Hi,

I tried installinig the Protobuf via both building from source and apt. When I tried to build ONNX optimizer from source, I got problems related to Protobuf. How do I specify Protobuf for installing ONNX optimizer? Thank you.

root@3235dac98ab6:/mnt# pip install -e .
Obtaining file:///mnt
Requirement already satisfied: onnx in /tmp/onnx (from onnxoptimizer==0.2.6) (1.9.0)
Requirement already satisfied: protobuf in /usr/local/lib/python3.8/dist-packages (from onnx->onnxoptimizer==0.2.6) (3.17.3)
Requirement already satisfied: numpy>=1.16.6 in /usr/local/lib/python3.8/dist-packages (from onnx->onnxoptimizer==0.2.6) (1.21.0)
Requirement already satisfied: six in /usr/local/lib/python3.8/dist-packages (from onnx->onnxoptimizer==0.2.6) (1.16.0)
Requirement already satisfied: typing-extensions>=3.6.2.1 in /usr/local/lib/python3.8/dist-packages (from onnx->onnxoptimizer==0.2.6) (3.10.0.0)
Installing collected packages: onnxoptimizer
  Running setup.py develop for onnxoptimizer
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/mnt/setup.py'"'"'; __file__='"'"'/mnt/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' develop --no-deps
         cwd: /mnt/
    Complete output (105 lines):
    running develop
    running build_py
    running create_version
    running cmake_build
    CMake Error at /usr/share/cmake-3.16/Modules/FindProtobuf.cmake:469 (file):
      file STRINGS file "/opt/conda/include/google/protobuf/stubs/common.h"
      cannot be read.
    Call Stack (most recent call first):
      third_party/onnx/CMakeLists.txt:172 (find_package)
    
    
    CMake Error at /usr/share/cmake-3.16/Modules/FindProtobuf.cmake:475 (math):
      math cannot parse the expression: " / 1000000": syntax error, unexpected
      exp_DIVIDE (2).
    Call Stack (most recent call first):
      third_party/onnx/CMakeLists.txt:172 (find_package)
    
    
    CMake Error at /usr/share/cmake-3.16/Modules/FindProtobuf.cmake:476 (math):
      math cannot parse the expression: " / 1000 % 1000": syntax error,
      unexpected exp_DIVIDE (2).
    Call Stack (most recent call first):
      third_party/onnx/CMakeLists.txt:172 (find_package)
    
    
    CMake Error at /usr/share/cmake-3.16/Modules/FindProtobuf.cmake:477 (math):
      math cannot parse the expression: " % 1000": syntax error, unexpected
      exp_MOD (2).
    Call Stack (most recent call first):
      third_party/onnx/CMakeLists.txt:172 (find_package)

Review setup.py and the failure at third_party/onnx/CMakeLists.txt:172, where CMake's FindProtobuf.cmake looks under /opt/conda/include. Reproduce the setup.py develop command and document how users should install or point to Protobuf for ONNX optimizer. Done means a clean reader can follow the instructions without this error.