Fix aten_stft ONNX spec violations

[justinchuby]

Summary

aten_stft emitted an ONNX STFT node that violated the operator spec in two ways (reported in #2942 by @bas-aarts):

1. frame_step / frame_length type mismatch. frame_step was built via op.Reshape(hop_length, [1]), producing a rank-1 tensor, while frame_length (n_fft) is a rank-0 scalar. The spec (T2 for both) requires them to share the same type/rank.
2. Signal rank. The ONNX STFT signal must be rank 3 ([batch, signal_length, 1] for real input). torch.stft accepts rank-1 or rank-2 signals, and the code only unsqueezed a rank-1 signal up to rank 2, leaving the signal one dimension short of the spec.

Fix

• Pass hop_length directly as frame_step (a rank-0 scalar, matching n_fft) and remove the frame_step_const = op.Reshape(...) line. hop_length is always a Python int in this trace-only function (input arg or n_fft // 4), so it is emitted as a rank-0 scalar exactly like n_fft.
• Add self = op.Unsqueeze(self, [-1]) after the existing batch-dim handling so the signal becomes rank 3 for both rank-1 and rank-2 inputs.

Rank trace (verified by inspecting the emitted STFT node)

Before → after, STFT inputs [signal, frame_step, window, frame_length]:

input	buggy rank	fixed rank	spec
signal	2	3	3
frame_step	1	0	scalar
window	1	1	1
frame_length	0	0	scalar

End-to-end shapes (fixed): rank-1 input [L] → batch unsqueeze [1, L] → trailing unsqueeze [1, L, 1] → STFT [1, frames, bins, 2] → Transpose [1, bins, frames, 2] → squeeze batch [bins, frames, 2]. Rank-2 input [B, L] → [B, L, 1] → [B, frames, bins, 2] → [B, bins, frames, 2]. Both match torch.stft's real output. The normalized and onesided paths are unaffected (only dtype/scaling, not rank).

Tests

• The existing OpInfo (ops.aten.stft) and _testing.assert_onnx_program value-comparison harnesses pass both before and after — they don't strictly enforce the STFT rank, which is why this spec bug went unnoticed.
• Added test_aten_stft_emits_spec_compliant_node (parameterized for rank-1 and rank-2 inputs) in tests/function_libs/torch_lib/e2e_ops_tests.py, which asserts the emitted STFT node's signal is rank 3 and frame_step/frame_length are both scalar. This test fails on the old code (2 != 3) and passes with the fix.

Ran:

• pytest tests/function_libs/torch_lib/ops_test.py -k stft → 2 passed, 1 skipped, 2 xfailed
• pytest tests/function_libs/torch_lib/e2e_ops_tests.py -k stft → 6 passed
• lintrunner -a → no lint issues

Fixes #2942

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.66%. Comparing base (029441f) to head (0f94339).
⚠️ Report is 12 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2943      +/-   ##
==========================================
+ Coverage   72.61%   72.66%   +0.04%     
==========================================
  Files         259      259              
  Lines       31597    31748     +151     
  Branches     2973     3005      +32     
==========================================
+ Hits        22945    23069     +124     
- Misses       7643     7660      +17     
- Partials     1009     1019      +10     

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