Arm backend: Add bf16 support for aten.index_select and aten.unfold_copy

[vacu9708]

Follow-up to #17097, which added BF16 support to the TOSA GATHER op.
aten.index_select and aten.unfold_copy both lower via TOSA GATHER
but their support checks were not updated at the time.

In both decompositions(DecomposeIndexSelectToGatherPass() and DecomposeUnfoldToGatherPass()),
the bf16 values tensor flows through dtype-agnostic reshape ops and tosa.GATHER, which accepts BF16.
The support check was the only blocker.

Op	bf16 before	bf16 after
aten.gather	✅	✅
aten.index.Tensor	✅	✅
aten.slice_copy	✅	✅
aten.index_select	❌	✅
aten.unfold_copy	❌	✅

Changes:

• index_select_support.py, unfold_copy_support.py: extend float branch
  to include bfloat16; add bf16 extension guard; update rejection message.
• test_index_select.py, test_unfold_copy.py: add isolated
  _tosa_FP_bf16 test functions using
  TosaPipelineFP(..., tosa_extensions=["bf16"]).

Test plan

test_index_select_tosa_FP_bf16 and test_unfold_copy_tosa_FP_bf16
exercise the bf16 path end-to-end through TosaPipelineFP with the bf16
extension enabled, following the same pattern of the existing
test_slice_tensor_tosa_FP_bf16 from #17492

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

[pytorch-bot]

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✅ You can merge normally! (4 Unrelated Failures)

As of commit 01ab271 with merge base b73df0b ():

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) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

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

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• ✅ login: vacu9708 / name: Youngsik Yang (b60efea)

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[zingo]

Hi, Thanks a lot for the fix! Feel free to submit/merge it.
If you cannot let us know and I/we can press the button!

[vacu9708]

@pytorchbot merge

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Mergebot is not configured for this repository. Please use the merge button provided by GitHub.

[vacu9708]

Thanks! @zingo It looks like mergebot isn't configured for this repository, and I don't have access to the GitHub merge button. Could you merge it for me?

[zingo]

Ill merge, thanks for the help.🙏
Failed test seems very unrelated.