VisionEncoderDecoderModel gradient checkpointing
### Feature request

Would love to be able to use gradient checkpointing on VisionEncoderDecoder model.
>>> model.gradient_checkpointing_enable() 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/conda/lib/python3.8/site-packages/transformers/modeling_utils.py", line 1418, in gradient_checkpointing_enable
    raise ValueError(f"{self.__class__.__name__} does not support gradient checkpointing.")
ValueError: VisionEncoderDecoderModel does not support gradient checkpointing.

### Motivation

Gradient checkpointing always helps increase the accessibility of larger models - HuggingFace is awesome!!!

### Your contribution

Happy to take a stab at this if someone can point me to a previous example of this working with an EncoderDecoder model.
@NielsRogge, have you seen such examples? :)<|||||>Here's a PR that added gradient checkpointing to T5: https://github.com/huggingface/transformers/pull/11353/files<|||||>Fixed per #18697