Add FlaxWhisperForAudioClassification model
Fixes #21779 
_The documentation is not available anymore as the PR was closed or merged._<|||||>cc @sanchit-gandhi <|||||>The test failures are appearing on this one. Let's fix them and re-merge!<|||||>We need to make two changes following the updates in #22954! First, we need to assign the attribute `gradient_checkpointing` to the class `FlaxWhisperForAudioClassificationModule`, similar to what we do for `FlaxWhisperForConditionalGeneration`:
https://github.com/huggingface/transformers/blob/a5741d7cb59f8a81d1f5fc7a6b106056d34f9969/src/transformers/models/whisper/modeling_flax_whisper.py#L1176
We then need to forward `self.gradient_checkpointing` to the encoder:
```diff
-         self.encoder = FlaxWhisperEncoder(config=self.config, dtype=self.dtype)
+         self.encoder = FlaxWhisperEncoder(config=self.config, dtype=self.dtype, gradient_checkpointing=self.gradient_checkpointing)
```
This will facilitate gradient checkpointing for the module!<|||||>@sgugger @sanchit-gandhi Done, all tests pass !