Rename SingleTPUPlugin to SingleTPUStrategy

[akihironitta]

What does this PR do?

Part of #10549.

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

Renamed SingleTPUPlugin to SingleTPUStrategy

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Did you have fun?

Make sure you had fun coding 🙃

cc @justusschock @awaelchli @akihironitta @Borda

[akihironitta]

@four4fish All I could find related to SingleTPU is only this occurrence bdfbc0d, but are there any other places I should fix?

[four4fish]

@four4fish All I could find related to SingleTPU is only this occurrence bdfbc0d, but are there any other places I should fix?

Nope, looks good to me! thanks