[tools/onnx-subgraph] Add readme.md for environment and existing code

tools/onnx_subgraph/README.md

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+# onnx_autosubgraph
+onnx-subgraph tool provides model auto partitionioning of onnx model to several sub models by
+operator, performance and model size limitations, with the order and input / output names of
+sub models.
+
+# How to build the onnx-subgraph
+## OS environment dependence
+     1. ubuntu >=20.04
+     2. GCC >= 9.4.0
+     3. cmake >= 3.10
+     4. python >= 3.8
+     5. apt-get install libprotobuf-dev protobuf-compiler libjsoncpp-dev
+
+## Python packages dependence
+    onnx                         1.16.0
+    onnxruntime                  1.18.1
+    onnxsim                      0.4.36
+    torch                        2.3.1
+    scikit-image
+    scikit-learn
+    pandas
+    tqdm
+
+## building the onnx-subgraph
+```bash
+    cd onnx-subgraph
+    mkdir build & cd build
+    cmake .. & make
+```
+    we can get following output at './build'
+```bash
+    scripts
+    ├── extract_onnx.py
+    └── test_model_download.sh
+    └── subgraphs_ios.txt
+```
+
+# How to use the onnx-subgraph
+## Pre-steps
+### Download the test AI models
+    1. 'bash scripts/test_model_download.sh', then "resnet-test.onnx" will be got in './build'
+    2. you can change to any other onnx files as your needs, or edit the download link in
+       "scripts/test_model_download.sh"
+
+## Parse the onnx model
+    note: 'subgraphs_ios.txt' will be generated in future code, suppose we already have it as
+    the example file now.
+
+## Split the onnx model to subgraphs
+```bash
+    python scripts/extract_onnx.py \
+              -s ./scripts/subgraphs_ios.txt \
+              -m ./resnet-test.onnx
+```
+    after extraction done, the subgraphs will be saved at './subgraphs'
+```bash
+    subgraphs
+    ├── CPUsubgraph0.onnx
+    └── CPUsubgraph1.onnx
+    ├── NPUsubgraph0.onnx
+    └── NPUsubgraph1.onnx
+```