[onnx_importer] Disambiguate empty string: optional none vs tensor name

[ziliangzl]

Fixes #4550
The NodeImporter cached torch.constant.none under _nv_map[""], matching ONNX's convention that an empty string in Node.input denotes an omitted optional input. Some producers (e.g. Microsoft SkipSimplifiedLayerNormalization) also bind real intermediate results to outputs whose names are the empty string. Each such output overwrote _nv_map[""], so later nodes that use "" for omitted optionals (e.g. GroupQueryAttention's trailing inputs) incorrectly received those tensor SSA values instead of torch.constant.none.

Behavior changes:

• Cache the shared none value under _OPTIONAL_NONE_CACHE_KEY instead of "".
• When resolving node inputs, treat input_name == "" as omitted optional: append get_none() and an empty onnx.TypeProto without indexing _nv_map[""].
• Register outputs named "" under unique keys _torch_mlir_onnx_importer_anon so multiple anonymous outputs do not overwrite each other.

Adds test/python/onnx_importer/test_empty_string_optional_inputs.py: minimal Identity -> custom op graph where optional inputs are "" and must import as %none operands, not tensor values stored under "".

Symptom fixed: GroupQueryAttention previously showed duplicated operands such as (%10#2, %10#2, %10#2) instead of (%none, %none, %none) for optional slots.

[ziliangzl]

Hi @IanWood1 , sorry to bother you. It looks like this repository doesn’t automatically assign reviewers, and I don’t have permission to request reviews from others. Would you be able to review this PR when you get a chance?

[IanWood1]

Thank you! The core idea looks right: empty string inputs should be imported as omitted optionals. I think there are still possible _nv_map collisions from storing importer private sentinel values in the map, so it would be good to address those before merging.

[ziliangzl]

@IanWood1 Hi, I’ve addressed the comments and updated the PR. Could you please take another look when you have a chance? If everything looks good, could you help merge it? Thanks!

[IanWood1]

LGTM!