Document what layerdrop does
### System Info

- `transformers` version: 4.28.1
- Platform: Windows-10-10.0.22621-SP0
- Python version: 3.11.2
- Huggingface_hub version: 0.14.1
- Safetensors version: not installed
- PyTorch version (GPU?): 2.0.1+cu117 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

### Who can help?

@sgugger 

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

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

### Reproduction

In `transformers/models/wav2vec2/configuration_wav2vec2.py` there is a parameter `layerdrop` in `__init__` which is not documented. This parameter is set (and overrides the default `0.1`) in the examples at `examples/pytorch/speech-recognition/README.md`, so it seems to be important.

### Expected behavior

Document `layerdrop`.
cc @sanchit-gandhi <|||||>Hey @RobertBaruch - good catch! It's indeed missing from the Wav2Vec2 config docstring. Would you like to open a PR to add this info? Easiest would be to copy the details from one of the existing configs where the info is present, e.g. OPT:
https://github.com/huggingface/transformers/blob/130e15429116689c9d747be2cdd8c4be7bb7e2bd/src/transformers/models/opt/configuration_opt.py#L70<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Closed via https://github.com/huggingface/transformers/pull/23691