Any documentation on how to add more layer types?

[joshterrill]

1. 使用环境（environment）

• OS: Mac
• OS Version: Sonoma 14.2.1

2. Github版本

• branch：master

3. 详细描述bug 情况 (Describe the bug)
A clear and concise description of what the bug is.

• issue type: convert failed
• original model： CoreML (espresso file)

4. 日志（Log）

expand_dims
Error: unsupported layer type: expand_dims

Is there any sort of documentation around that I can reference if I want to add additional supported layer types to tools/onnx2tnn/onnx-coreml/coreml2onnx.py?

[joshterrill]

I figured out how to modify the existing coreml2onnx script to support blob_weights_f16 and softmax. There were also a few bugs that I fixed where the shapes of some of the conv layers that were being generated were incorrect. Would you take a PR to look at it?