[`RWKV`] Add Gradient Checkpointing support for RWKV
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/24831

As stated by the issue above, technically there is no reason to not support GC for RWKV model

cc @sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>This snippet worked on my end:

```python
import torch
from transformers import RwkvForCausalLM

model = RwkvForCausalLM.from_pretrained("RWKV/rwkv-4-169m-pile").to(0)
model.train()
model.gradient_checkpointing_enable()

dummy_input = torch.LongTensor([[1, 2, 3, 4]]).to(0)

outputs = model(dummy_input)
logits = outputs.logits

loss = logits.mean()
loss.backward()
```
I will assume things are working (+ the CI should be triggered once the model has `supports_gradient_checkpointing = True`) , merging! 