[BE] Make ScalingGranularity module level so it can be rendered in API ref on docsite

[danielvegamyhre]

See #2313 for PR adding float8 training API ref to docsite

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/2314

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❌ 2 New Failures

As of commit 9171e84 with merge base d963a88 ():

NEW FAILURES - The following jobs have failed:

• Code Analysis with Ruff / build (3.9) (gh)
  Process completed with exit code 1.
• PR Label Check / Check PR Labels (gh)
  Process completed with exit code 1.

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