Support installation in editable mode

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
```