Fix scalar (single element tensor) binary ops on HiFi

[mcremon-meta]

Summary:
As titled, when introduced I used dim but I should have used numel. Fixed here.
Also need to update the quantizer for the add case, will do in a separate diff.

Differential Revision: D71667998

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/9523

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❌ 3 New Failures

As of commit 6f57e08 with merge base fb24360 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for examples/models/moshi/mimi/test_mimi.py:
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 8f9b5c7167b0886a535db74f43e619243fa11114df2cea9a6fbc06b2dd3e29ea /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D71667998

[zonglinpeng]

Ah good catch