Merge back to back slices on the same dim

[DrJessop]

Summary:
If we have back to back slices on the same dimension, we can remove the top slice and just perform the second one. RemoveOrReplacePassInterface will handle a whole cascade if it exists.

Handles negative start/end indices by canonicalizing them, and negative dims by normalizing against ndim. Clamps the merged end to the parent's end to handle child ranges that exceed the parent output size.

Differential Revision: D102425537

[pytorch-bot]

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❌ 2 Cancelled Jobs, 2 Unrelated Failures

As of commit 0204b9e with merge base 2330652 ():

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• periodic / gather-models (gh)
• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.

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]

@DrJessop has exported this pull request. If you are a Meta employee, you can view the originating Diff in D102425537.

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