Rename GPUAccelerator to CUDAAccelerator

[justusschock]

What does this PR do?

As discussed in #13102 this renames the current GPUAccelerator to CUDAAccelerator.
Introduces strategy='cuda' but retains strategy='gpu' for this PR temporarily

In a follow-up, I will re-route accelerator='gpu' to support cuda and mps devices (as also discussed in #13102 )

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

Yes, deprecates the current GPU Accelerator

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Make sure you had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @tchaton @kaushikb11 @akihironitta

[akihironitta]

LGTM! 🚀 Should we preserve the git history with "rebase&merge"? (If we do, will need to clean up the commits)

[justusschock]

@akihironitta wouldnt we just squash merge it? That should be fine, no?

[rohitgr7]

Agreed with @akihironitta !

we should do rebase and merge with 2 commits to preserve the git history. squash and merge will not keep it.