Update deepspeed activation checkpointing docs

[avivbrokman]

📚 Documentation

In your documentation, 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

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