Fix privateuse1 backend name case

[shink]

Problem

get_privateuse1_backend(bool lower_case) always returns a lower case name and lower_case is not used.

cc: @FFFrog

[pytorch-bot]

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[albanD]

Sounds good as is!
No need to add it to the python API unless you need it!

[FFFrog]

@pytorchbot merge

[pytorchmergebot]

Merge failed

Reason: This PR needs a release notes: label
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[FFFrog]

@pytorchbot label "topic: not user facing"

[FFFrog]

@pytorchbot merge

[pytorchmergebot]

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