Raise exception for strategy=ddp_cpu|tpu_spawn

[carmocca]

What does this PR do?

Part of #9932

No usage yet:

$ grep -iIrn 'stategy="ddp_cpu' pytorch_lightning docs tests
$ grep -iIrn 'stategy="tpu_spawn' pytorch_lightning docs tests

Does your PR introduce any breaking changes? If yes, please list them.

None, strategy is unreleased.

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[kaushikb11]

LGTM!

[tchaton]

LGMT !