Fix SequenceEmpty dtype parameter ignored in eager mode

[PratikWayase]

Summary

Fix SequenceEmpty dtype parameter being ignored in eager/trace mode by introducing a typed runtime representation for sequences. Fixes #1562.

Changes

As noted by @gramalingam in the issue, the root cause is that we reuse Python lists for runtime sequence values, and when the list is empty, we cannot infer a suitable ONNX type. This PR implements the first proposed fix: using a runtime representation that combines a type with the value.

• Introduce _TypedSequence, a list subclass in onnxscript/_internal/typed_sequence.py that preserves ONNX dtype information for empty sequences
• Modify BaseEvaluator.eval_op in onnxscript/_internal/evaluator.py to wrap SequenceEmpty results with _TypedSequence using the provided dtype attribute (defaults to FLOAT if None)
• Modify _onnxscript_to_numpy_value in onnxscript/_internal/evaluator.py to preserve the _TypedSequence wrapper (instead of downcasting to a plain list) when preparing inputs for the ONNX runtime
• Modify value_to_type_proto in onnxscript/_internal/utils.py to extract the dtype from _TypedSequence when inferring the ONNX TypeProto

Tests

Added OpInfo-style regression test test_sequence_empty_preserves_dtype covering:

• FLOAT16 dtype
• DOUBLE dtype
• INT64 dtype

Additionally verified edge cases:

• Missing dtype attribute (correctly defaults to FLOAT)
• Multiple SequenceInsert operations on the same typed sequence
• SequenceAt on a typed sequence

Files:

• onnxscript/_internal/typed_sequence.py (new)
• onnxscript/_internal/evaluator.py
• onnxscript/_internal/utils.py
• tests/eager_mode_test.py

Validation

python -m pytest tests/eager_mode_test.py -k "test_sequence_empty_preserves_dtype" -v

Passed:

tests/eager_mode_test.py::EagerModeTest_0_reference_runtime::test_sequence_empty_preserves_dtype PASSED
tests/eager_mode_test.py::EagerModeTest_1_onnxruntime::test_sequence_empty_preserves_dtype PASSED
2 passed