Depricate xpu_backend for ddp_backend (#23085)
* Depricate xpu_backend for ddp_backend
* Typo
* Only do a minor deprecation, no need for major
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>