Add table of contents to Python samples

[natke]

No description provided.

[KevinH48264]

Looks good

[natke]

Anyone know why the CI is failing? @snnn @edgchen1

 Traceback (most recent call last):
2022-06-03T18:51:34.4150440Z   File "/Users/runner/work/1/s/mobile/examples/basic_usage/model/single_add_gen.py", line 1, in <module>
2022-06-03T18:51:34.4214450Z     import onnx
2022-06-03T18:51:34.4246740Z   File "/Users/runner/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/onnx/__init__.py", line 11, in <module>
2022-06-03T18:51:34.4266000Z     from onnx.external_data_helper import load_external_data_for_model, write_external_data_tensors, convert_model_to_external_data
2022-06-03T18:51:34.4304670Z   File "/Users/runner/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/onnx/external_data_helper.py", line 14, in <module>
2022-06-03T18:51:34.4410450Z     from .onnx_pb import TensorProto, ModelProto
2022-06-03T18:51:34.4511130Z   File "/Users/runner/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/onnx/onnx_pb.py", line 8, in <module>
2022-06-03T18:51:34.4563450Z     from .onnx_ml_pb2 import *  # noqa
2022-06-03T18:51:34.4665740Z   File "/Users/runner/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/onnx/onnx_ml_pb2.py", line 37, in <module>
2022-06-03T18:51:34.4760450Z     create_key=_descriptor._internal_create_key),
2022-06-03T18:51:34.4815600Z   File "/Users/runner/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/google/protobuf/descriptor.py", line 755, in __new__
2022-06-03T18:51:34.4844640Z     _message.Message._CheckCalledFromGeneratedFile()
2022-06-03T18:51:34.4855810Z TypeError: Descriptors cannot not be created directly.
2022-06-03T18:51:34.4879600Z If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
2022-06-03T18:51:34.4908350Z If you cannot immediately regenerate your protos, some other possible workarounds are:
2022-06-03T18:51:34.4917580Z  1. Downgrade the protobuf package to 3.20.x or lower.
2022-06-03T18:51:34.4927720Z  2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).

[edgchen1]

Looks like this is the issue: onnx/onnx#4222
ONNX didn't specify an upper bound for the version of its Protobuf dependency before and there was a breaking change in a new Protobuf version.

#119 should address it.