Export aten::relu6 as a single Clip op

[Copilot]

aten::relu6 was exported as a Relu + Min pair, which relies on downstream fusion to become a single clamp. ONNX has Clip, which expresses this directly.

Changes

• onnxscript/function_libs/torch_lib/ops/nn.py: aten_relu6 now emits Clip(self, 0, 6) with bounds cast to the input dtype.

@torch_op("aten::relu6", trace_only=True)
def aten_relu6(self: TReal) -> TReal:
    """relu6(Tensor self) -> Tensor"""

    zero = op.CastLike(op.Constant(value_int=0), self)
    six = op.CastLike(op.Constant(value_int=6), self)
    return op.Clip(self, zero, six)

Benchmark

Measured on onnxruntime CPU EP (float32, 500 iterations, mean per-run latency) per the issue's request to confirm which form is faster:

shape	Clip	Relu + Min
(1, 64, 56, 56)	15.5 µs	67.3 µs
(64, 1024)	8.3 µs	18.8 µs

Clip is ~2–4x faster unfused, so the single-op form is preferable even before any graph optimization.

• Fixes Improved ONNX export for relu6 layer #2986

[Copilot]

Pull request overview

This PR improves the ONNX export of aten::relu6 by emitting a single Clip op (with min=0, max=6) instead of the previous Relu + Min pair, aligning the export with ONNX’s native clamp representation and avoiding reliance on downstream fusion.

Changes:

• Update aten_relu6 to return op.Clip(self, zero, six) rather than op.Min(op.Relu(self), six).
• Ensure Clip bounds are cast to the input dtype via CastLike.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (4cd022b) to head (33bda6b).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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##             main    #2987   +/-   ##
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  Coverage   72.63%   72.63%           
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  Files         265      265           
  Lines       32204    32205    +1     
  Branches     3041     3041           
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+ Hits        23391    23392    +1     
  Misses       7779     7779           
  Partials     1034     1034           

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