[Flax Examples] Seq2Seq ASR Fine-Tuning Script
# What does this PR do?

Can be used to fine-tune Flax Whisper for speech recognition.

Tested and verified as working with the following (dummy) config:
```
run_flax_speech_recognition_seq2seq.py \
            --model_name_or_path openai/whisper-tiny.en \
            --dataset_name hf-internal-testing/librispeech_asr_dummy \
            --dataset_config clean \
            --train_split_name validation \
            --eval_split_name validation \
            --output_dir whisper-tiny-ft-dummy \
            --overwrite_output_dir \
            --num_train_epochs=2 \
            --max_train_samples 10 \
            --max_eval_samples 10 \
            --warmup_steps=8 \
            --do_train \
            --do_eval \
            --learning_rate=2e-4 \
            --per_device_train_batch_size=2 \
            --per_device_eval_batch_size=1 \
            --predict_with_generate
```

Will add a README with preliminary training configs / results later this week after doing a full fine-tuning run.

cc @peregilk @andyehrenberg for interest
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21764). All of your documentation changes will be reflected on that endpoint.<|||||>@sanchit-gandhi @andyehrenberg 

We have made a version of this script will support streaming and training on the TPU pods.

The current version of the script is available here:
[https://github.com/NbAiLab/nb-whisper/blob/main/run_flax_speech_recognition_seq2seq_streaming.py](https://github.com/NbAiLab/nb-whisper/blob/main/run_flax_speech_recognition_seq2seq_streaming.py)

We are however struggling with a bug at the moment. The script seems to work for training the Tiny models on multiple pod sizes. Both for scaling for speed and for increasing the batch size. All the other model sizes (small, base, medium, large) also works on the single TPU v4-8. However, training on the non-Tiny-model sizes runs for a few steps then freezes. 

If anyone have any idea about this could be happening, I really appreciate it.