[ONNX] Fix roi_align ONNX export (#3355)

* add tests

* fix bug

* remove tests

* fix comment

* fix comment

* add warning

* fix syntax error

* fix python lint
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

test/test_onnx.py

@@ -129,6 +129,11 @@ class ONNXExporterTester(unittest.TestCase):
         model = ops.RoIAlign((5, 5), 1, 2)
         self.run_model(model, [(x, single_roi)])
 
+        x = torch.rand(1, 1, 10, 10, dtype=torch.float32)
+        single_roi = torch.tensor([[0, 0, 0, 4, 4]], dtype=torch.float32)
+        model = ops.RoIAlign((5, 5), 1, -1)
+        self.run_model(model, [(x, single_roi)])
+
     def test_roi_align_aligned(self):
         x = torch.rand(1, 1, 10, 10, dtype=torch.float32)
         single_roi = torch.tensor([[0, 1.5, 1.5, 3, 3]], dtype=torch.float32)
@@ -150,6 +155,11 @@ class ONNXExporterTester(unittest.TestCase):
         model = ops.RoIAlign((2, 2), 2.5, 0, aligned=True)
         self.run_model(model, [(x, single_roi)])
 
+        x = torch.rand(1, 1, 10, 10, dtype=torch.float32)
+        single_roi = torch.tensor([[0, 0.2, 0.3, 4.5, 3.5]], dtype=torch.float32)
+        model = ops.RoIAlign((2, 2), 2.5, -1, aligned=True)
+        self.run_model(model, [(x, single_roi)])
+
     @unittest.skip  # Issue in exporting ROIAlign with aligned = True for malformed boxes
     def test_roi_align_malformed_boxes(self):
         x = torch.randn(1, 1, 10, 10, dtype=torch.float32)

torchvision/ops/_register_onnx_ops.py

@@ -29,6 +29,12 @@ def _register_custom_op():
                           " ONNX forces ROIs to be 1x1 or larger.")
             scale = torch.tensor(0.5 / spatial_scale).to(dtype=torch.float)
             rois = g.op("Sub", rois, scale)
+
+        # ONNX doesn't support negative sampling_ratio
+        if sampling_ratio < 0:
+            warnings.warn("ONNX doesn't support negative sampling ratio,"
+                          "therefore is is set to 0 in order to be exported.")
+            sampling_ratio = 0
         return g.op('RoiAlign', input, rois, batch_indices, spatial_scale_f=spatial_scale,
                     output_height_i=pooled_height, output_width_i=pooled_width, sampling_ratio_i=sampling_ratio)