Arm backend: Add FP8 support for gather/scatter-based composite ops

[YufengShi-dudu]

Support FP8 tensors for the following composite ops:

• TOSA GATHER: embedding, index_select, index.Tensor, unfold_copy
• TOSA SCATTER: index_put, index_copy, slice_scatter

Run all FP8 tests through the TOSA reference model. For ops without eager CPU FP8 support, only execute the TOSA reference model; otherwise keep the default output comparison against eager.

Change-Id: I3d81cd6dd426f16b5f2db8937228cad12184b6a6

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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❌ 6 New Failures, 1 Pending, 1 Unrelated Failure

As of commit c4aab47 with merge base e28ef13 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  Input required and not supplied: aws-region
• Cadence Build & Test / vision-build / vision (gh)
  Input required and not supplied: aws-region
• pull / test-qnn-models-linux (mv2) / linux-job (gh)
  RuntimeError: Command docker exec -t b2f9ca6b30b508f6f353838ce70d0ad1e3a9c476fd04c42787ae5328f1351fc5 /exec failed with exit code 92
• pull / unittest / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch
• trunk / test-qnn-model (fp32, conv_former) / linux-job (gh)
  RuntimeError: Command docker exec -t 9841488caeafba73e1e74e70d5d1ad33df2501ff0b67554e41a9964edf26ce9c /exec failed with exit code 92
• trunk / unittest-release / macos / macos-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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