Use get_arg in fusion pass

[mcremon-meta]

Summary: As titled. It makes things easier to read, and prevents issues with arg ordering when making changes.

Differential Revision: D99233320

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 6c8f631 with merge base 3a6314e ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@mcremon-meta has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99233320.

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