Support sliding-window layers on the smart_mask cache ring

[YIWENX14]

Summary: Sliding-window (local) attention layers previously worked only with the shift_pointer KV cache layout. This makes them work with smart_mask as well, so chunked prefill matches single-chunk prefill under either layout.

Differential Revision: D116524534

[pytorch-bot]

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❌ 2 New Failures, 1 Unclassified Failure

As of commit b381b90 with merge base ed2d9ec ():

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[meta-codesync]

@YIWENX14 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D116524534.