onnx-subgraph tool step by step submitting initial PR

[chenyx113]

initial PR of onnx-subgraph tool.
related issue of: #14534
historical draft PR:
#14613

I will submit this tool from zero, this submitting include 5 files:

1. extract_onnx.py is the code for how to split onnx model to sub models by using given config file
2. readme.md just include the basic environment setting and how to use extract_onnx.py
3. CMakeLists.txt only include the python code & related environment installing for extract_onnx.py
4. subgraphs_ios.txt is an example config file for testing, which will be generated in future code, at that time, this file will be removed
5. test_model_download.sh is used for downloading the test AI model, it will run at ./build path, will not effect git status

this is minimum splitting I can do for the initial PR, please help review, thank you

ONE-DCO-1.0-Signed-off-by: Youxin Chen [email protected]

[seanshpark]

Q) what is the purpose of this draft?

[chenyx113]

Q) what is the purpose of this draft?

there are 2 purposes of this draft:

1. initial submitting to for onnx-subgraph tool, include CMakeLists, Readme.md and other testing dependency
2. to tell the beginning of story: how to split onnx model to sub models by using "given config file" with extract_onnx.py

in future code, we will show how to test the sub models, how to auto generate the "given config file" for each onnx model.

[seanshpark]

#14593 (comment)

Please post a single context of changes per PR.
I think you posted 5 commits in a PR.

[chenyx113]

#14593 (comment)

Please post a single context of changes per PR. I think you posted 5 commits in a PR.

OK，I will submit a new one in a clear branch sync with master branch, because master is updating continuously, I add 2 rebases additionally, is it ok to add rebase after PR submitting? to avoid the merge fatal issue