Raise inoperative error guards in ExportSession, XNNPACK node_visitor, and Samsung conv1d

[AxelNoun]

Summary

Four error-guard sites construct a failure that cannot actually fire. This PR inserts the missing raise (or replaces an always-true assert with the file's existing check_or_raise) so the documented failure mode is the one callers see.

File	What happens today	Reachability
export/export.py ExportSession.print_delegation_info, empty lowering-stage set	RuntimeError(...) is built and discarded; lowering_stage[0] then raises IndexError: list index out of range	Reachable. ExportSession is in executorch.export.__all__ and is @experimental. Calling print_delegation_info() on a session whose pipeline has no lowering stage hits this path.
export/export.py print_delegation_info, lowering stage listed but not run	RuntimeError(...) is built and discarded; None.get_context(...) then raises AttributeError: 'NoneType' object has no attribute 'get_context'	Reachable. A pipeline that lists TO_EDGE_TRANSFORM_AND_LOWER (or TO_BACKEND) but has not run that stage hits this path.
backends/xnnpack/operators/node_visitor.py per-channel axis else	assert f"Unsupported weight per channel quantization axis ..." — the f-string is always truthy, so the assertion never fires. An assert would also disappear under python -O.	Not reachable on a normal export. The only call that passes swap_in_out_for_weights=True is backends/xnnpack/operators/op_conv2d.py:102, and that visitor already check_or_raises the axis to 0 (depthwise) or 1 (transpose) before define_tensor. This change restores a guard that can actually raise if that else is ever entered; it does not fix a mis-serialized .pte on the current conv2d path.
backends/samsung/_passes/conv1d_to_conv2d.py update_kernel else	RuntimeError("Weight of 1d conv should be constant tensor or Parameter obj") is built and discarded; weight_node.meta["val"] is still unsqueezed.	Not reachable on a normal Parameter / lifted-constant conv1d export. Those two cases are handled by the preceding branches. The else is the leftover for a weight that is none of those. This change only makes that documented RuntimeError actually raise if the branch is entered.

print_delegation_info also corrects the message "atleast" → "at least".

Test plan

Tests are included only for the two reachable sites (F3a). There is intentionally no test for the XNNPACK else (F2) and none for the Samsung else (F3b): both are unreachable on a normal export, and a test that injects that state would exercise a situation the production callers do not produce.

export/tests/test_print_delegation_info.py covers the two ExportSession guards (export/tests is already in pytest.ini testpaths):

pytest export/tests/test_print_delegation_info.py

On 6755ea388ce6d2249d26f568c286a153ccba4883:

• unpatched: IndexError: list index out of range and AttributeError: 'NoneType' object has no attribute 'get_context'
• patched: both tests pass with RuntimeError and the documented messages

F2: op_conv2d.py already rejects axis ∉ {0, 1} before define_tensor. No AST test and no injected-axis test is in this PR.

F3b: a normal Parameter / lifted-constant conv1d does not enter the else. No monkeypatch test is in this PR. The change only restores the RuntimeError the source already spelled out, if that branch is ever entered.

A separate, two-line cleanup (duplicate dataclass fields in examples/models/llama/model_args.py and exir/serde/schema.py) is available as differe-f4.patch if a follow-up PR is wanted; it is not part of this change.

[pytorch-bot]

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⚠️ 14 Awaiting Approval

As of commit 95b4eff with merge base c27baa8 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test Cortex-M Backend (gh)
• Test NXP Backend (gh)
• Test QNN Backend (gh)
• Test QNN Windows MSVC build (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[AxelNoun]

@pytorchbot label "release notes: none"

@pytorchbot label "release notes: none"