[Trainer] use deepspeed SP process group when Accelerate doesn’t build a mesh
Author: kashif
Pull request: yes

# What does this PR do?

With this change: https://github.com/huggingface/accelerate/pull/3916 there is no torch device mesh when sp_backend="deepspeed", and transformers currently assumes it exists.
  That’s why you get:

```
  sp_group = self.accelerator.torch_device_mesh["sp"].get_group()
  TypeError: 'NoneType' object is not subscriptable
```

With this change, it uses DeepSpeed’s SP process group when Accelerate doesn’t build a mesh.

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Comments:
cc @stas00 if you can kindly check?

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