Add activation checkpointing to DPO and GRPO

### Feature request

Currently TRL only implemented activation checkpointing for SFT (https://github.com/huggingface/trl/pull/2954), can we also add it to DPO and GRPO? Will the same code changes from SFT also work for those?

### Motivation

Since SFT has activation checkpointing, it would be good that other training algorithms have the same feature.

### Your contribution

If the same code changes from SFT also work for DPO and GRPO, I can submit a PR.