onnx-subgraph initial PR, step by step submitting with small PRs

[chenyx113]

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

#14618

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 yx113.chen@samsung.com

[seanshpark]

I don't know what's the difference between #14618 as a result.
This also has 5 file changes and lots of context of changes. (I didn't check in detail)

Please put ONLY with CMakeLists.txt file in a PR.
With that PR, I will check in my local that the build is OK according to your README file how to build.
If the build is success, I'll land that PR.
And prepare a Workflow to check following PR build is success or not.

[chenyx113]

I don't know what's the difference between #14618 as a result. This also has 5 file changes and lots of context of changes. (I didn't check in detail)

Please put ONLY with CMakeLists.txt file in a PR. With that PR, I will check in my local that the build is OK according to your README file how to build. If the build is success, I'll land that PR. And prepare a Workflow to check following PR build is success or not.

OK, I understand, you will reference the full changes historical PR, I will re-submit