Support installation in editable mode

[guangy10]

Issue:
If we install optimum-executorch in editable mode:

git clone https://github.com/huggingface/optimum-executorch.git
cd optimum-executorch
pip install -e .

This can run through successfully.

optimum              1.24.0
optimum-executorch   0.0.0.dev0         /Users/guangyang/optimum-executorch

However, when run optimum-cli export executorch --help, it will end with error:

usage: optimum-cli export [-h] {onnx,tflite} ...
optimum-cli export: error: argument {onnx,tflite}: invalid choice: 'executorch' (choose from 'onnx', 'tflite')
(optimum-executorch)[guangyang-mbp (fix_readme) *]: ~/optimum-executorch$ optimum-cli export executorch --help