[ET-VK] Insert prepack nodes for constant primary inputs of prepacking ops

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK] Insert prepack nodes for constant primary inputs of prepacking ops #17850
• [ET-VK] Fix mixed-dtype binary ops and comparison op padding bugs #17849
• [ET-VK] Fix softmax NaN and depthwise conv correctness bugs #17848

The insert_prepack_nodes pass was skipping prepack node insertion for all
constant tensor args of ops with supports_prepacking=True. However, these ops
only handle prepacking for weight/bias tensors internally; the primary input
tensor is still expected to be a GPU tensor. If the primary input happens to be
a constant tensor (serialized as TensorRef), the op throws an exception at
runtime.

Fix this by detecting the primary input index directly in insert_prepack_nodes.
Most prepacking ops have the primary input at arg 0, but embedding uses arg 1
since its signature is embedding(weight, indices, ...). The pass now checks
whether a constant tensor is used as the primary input of a prepacking op, and
if so, still inserts a prepack node for it.

Differential Revision: D95217949

[pytorch-bot]

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❌ 1 New Failure, 4 Unrelated Failures

As of commit 2d65b78 with merge base 1a75394 ():

NEW FAILURE - The following job has failed:

• Test Metal Backend / test-model-metal-e2e (openai, whisper-large-v3-turbo, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

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 / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, non-quantized) / windows-job (gh) (trunk failure)
  Process completed with exit code 1.
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-weight-only) / windows-job (gh) (trunk failure)
  Process completed with exit code 1.

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