Fix optimum-cli export executorch

[guangy10]

System Info

Issue:

Run the following cmd on latest optimum
`optimum-cli export executorch --model hf-internal-testing/tiny-random-LlamaForCausalLM --task text-generation --recipe xnnpack --output_dir meta_llama3_2_1b`
will get errors:

usage: optimum-cli export [-h] {onnx,tflite} ...
optimum-cli export: error: argument {onnx,tflite}: invalid choice: 'executorch' (choose from 'onnx', 'tflite')


This is a regression due to the repo migration. We should add executorch back to the CLI.

Who can help?

@echarlaix

Information

• The official example scripts
• My own modified scripts

Tasks

• An officially supported task in the examples folder (such as GLUE/SQuAD, ...)
• My own task or dataset (give details below)

Reproduction (minimal, reproducible, runnable)

Simple run optimum-cli export executorch --help can reproduce

Expected behavior

The CLI should export the model to ExecuTorch

[echarlaix]

Discussed in huggingface/optimum-executorch#7