Python pnnx with pnnx binary

[Hideousmon]

Python wheels have been created for PNNX using Pybind with CI.

You can find the release test, combined with pytest, at the following location: https://github.com/Hideousmon/ncnn/actions/runs/6433620002 .

The pytest files are available here: https://github.com/Hideousmon/ncnn/tree/python-pnnx-with-pybind/tools/pnnx/python/tests .

A basic documentation can be found here: https://github.com/Hideousmon/ncnn/blob/python-pnnx-with-pybind/tools/pnnx/python/README.md , which can also be found in this pull request.

The codes for Pypi publishment in the .yml file are currently commented out. A new API Token for PNNX project may need to be created first.

Note:

1. The pnnx2onnx feature is currently disabled, primarily due to limitations in my PC's environment for building protobuf.
2. Presently, we offer support exclusively for x86_64 (amd64) architecture. This limitation is primarily due to the significant amount of additional work required to expand compatibility.

[Hideousmon]

TO DO:

• enable pnnx2onnx feature.
• fix wheels repairing on linux & macos.
• check workability with different versions of pytorch after wheels repairing.
• expand compatibility for different platforms.

[Hideousmon]

The attempt with pybind is aborted because of a challenging error for me under GCC, which is a segmentation fault when simultaneously using the pre-compiled pnnx shared library alongside the pytorch.
Now the python pnnx wheel contains a pnnx binary to avoid the utilization of shared library. The release test and functions test can be found at: https://github.com/Hideousmon/pnnx/actions/runs/6691173863
A corresponding pull request is on pnnx/pnnx for ci release: pnnx/pnnx#94 .

[nihui]

虽然还有点细节的问题，先merge了！
Thanks for your contribution !