Update deepspeed activation checkpointing docs

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### 📚 Documentation

In your [documentation](https://lightning.ai/docs/pytorch/stable/advanced/model_parallel.html#deepspeed-activation-checkpointing), you refer to the function `deepspeed.checkpointing.checkpoint`, but it looks like it does not exist (anymore?). Can you update that section?

While we're at it, can you provide a more common use-case as an example? The guide warns against wrapping an entire model, but having a pretrained language model from `transformers` is probably the most common use case. What if someone is just using GPT-2 or T5 with no further mathematical layers? What should get wrapped then?

cc @borda @awaelchli

Start with the linked DeepSpeed activation checkpointing section in the documentation and verify whether deepspeed.checkpointing.checkpoint still exists. Compare the guidance with common GPT-2 and T5 usage, then update the section so the API reference and wrapping recommendation are accurate for those cases.

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