Rename the qnn demo backend

[cccclai]

Summary: Suspect the failure https://github.com/pytorch/pytorch/actions/runs/19547462483/job/55989739476 is due to using different QnnBackend implementation. Rename this demo backend to a demo backend name

Differential Revision: D87586567

[pytorch-bot]

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✅ You can merge normally! (6 Unrelated Failures)

As of commit feb1edb with merge base b4d72f1 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (emformer_transcribe, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (resnet50, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-static-llama-qnn-linux (stories_260k_bc) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-vulkan-models-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-wasm-bindings (--enable-etdump) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[meta-codesync]

@cccclai has exported this pull request. If you are a Meta employee, you can view the originating Diff in D87586567.

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