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

[Copilot]

Pull request overview

This PR fixes eager/trace-mode handling of SequenceEmpty(dtype=...) by introducing a typed runtime sequence representation so empty sequences retain an ONNX element dtype and can safely flow into later sequence ops (e.g., SequenceInsert).

Changes:

• Added _TypedSequence (a list subclass carrying an onnx_dtype) as an internal runtime representation for sequences with explicit element dtype.
• Updated eager-mode evaluation to wrap SequenceEmpty outputs in _TypedSequence, and to preserve _TypedSequence when converting values for runtime execution and type inference.
• Added an eager-mode regression test covering FLOAT16, DOUBLE, and INT64 dtype for SequenceEmpty + SequenceInsert.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/eager_mode_test.py	Adds regression coverage ensuring SequenceEmpty(dtype=...) prevents SequenceInsert elem-type mismatches in eager mode.
onnxscript/_internal/utils.py	Updates type inference to extract ONNX element dtype from _TypedSequence instead of defaulting empty lists to float.
onnxscript/_internal/typed_sequence.py	Introduces _TypedSequence wrapper that carries onnx_dtype for empty (and subsequent) sequence values.
onnxscript/_internal/evaluator.py	Wraps SequenceEmpty outputs in _TypedSequence and preserves _TypedSequence during conversion for runtime execution.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.63%. Comparing base (d3489d4) to head (c8258c8).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/_internal/typed_sequence.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2949      +/-   ##
==========================================
+ Coverage   72.61%   72.63%   +0.01%     
==========================================
  Files         263      264       +1     
  Lines       32034    32057      +23     
  Branches     3013     3016       +3     
==========================================
+ Hits        23263    23283      +20     
- Misses       7748     7750       +2     
- Partials     1023     1024       +1     

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