onnx export of per channel fake quantize functions (pytorch#42835)

Summary:
Fixes pytorch#39502

This PR adds support for exporting **fake_quantize_per_channel_affine** to a pair of QuantizeLinear and DequantizeLinear. Per tensor support was added by PR pytorch#39738.

`axis` attribute of QuantizeLinear and DequantizeLinear, which is required for per channel support, is added in opset13 added by onnx/onnx#2772.

[update 1/20/2021]: opset13 is being supported on master, the added function is now properly tested. Code also rebased to new master.

The function is also tested offline with the following code
```python
import torch
from torch import quantization

from torchvision import models
qat_resnet18 = models.resnet18(pretrained=True).eval().cuda()

qat_resnet18.qconfig = quantization.QConfig(
    activation=quantization.default_fake_quant, weight=quantization.default_per_channel_weight_fake_quant)
quantization.prepare_qat(qat_resnet18, inplace=True)
qat_resnet18.apply(quantization.enable_observer)
qat_resnet18.apply(quantization.enable_fake_quant)

dummy_input = torch.randn(16, 3, 224, 224).cuda()
_ = qat_resnet18(dummy_input)
for module in qat_resnet18.modules():
    if isinstance(module, quantization.FakeQuantize):
        module.calculate_qparams()
qat_resnet18.apply(quantization.disable_observer)

qat_resnet18.cuda()

input_names = [ "actual_input_1" ]
output_names = [ "output1" ]

torch.onnx.export(qat_resnet18, dummy_input, "quant_model.onnx", verbose=True, opset_version=13)
```
It can generate the desired graph.

Pull Request resolved: pytorch#42835

Reviewed By: houseroad

Differential Revision: D26293823

Pulled By: SplitInfinity

fbshipit-source-id: 300498a2e24b7731b12fa2fbdea4e73dde80e7ea

Author: skyw

Diff for: test/onnx/test_models.py

@@ -182,7 +182,7 @@ def test_fake_quant(self):
         self.exportTest(toC(FakeQuantNet()), toC(x))
 
     @skipIfUnsupportedMinOpsetVersion(10)
-    def test_qat_resnet(self):
+    def test_qat_resnet_pertensor(self):
         # Quantize ResNet50 model
         x = Variable(torch.randn(BATCH_SIZE, 3, 224, 224).fill_(1.0))
         qat_resnet50 = resnet50()
@@ -202,6 +202,27 @@ def test_qat_resnet(self):
 
         self.exportTest(toC(qat_resnet50), toC(x))
 
+    @skipIfUnsupportedMinOpsetVersion(13)
+    def test_qat_resnet_per_channel(self):
+        # Quantize ResNet50 model
+        x = torch.randn(BATCH_SIZE, 3, 224, 224).fill_(1.0)
+        qat_resnet50 = resnet50()
+
+        qat_resnet50.qconfig = quantization.QConfig(
+            activation=quantization.default_fake_quant,
+            weight=quantization.default_per_channel_weight_fake_quant)
+        quantization.prepare_qat(qat_resnet50, inplace=True)
+        qat_resnet50.apply(torch.quantization.enable_observer)
+        qat_resnet50.apply(torch.quantization.enable_fake_quant)
+
+        _ = qat_resnet50(x)
+        for module in qat_resnet50.modules():
+            if isinstance(module, quantization.FakeQuantize):
+                module.calculate_qparams()
+        qat_resnet50.apply(torch.quantization.disable_observer)
+
+        self.exportTest(toC(qat_resnet50), toC(x))
+
     @disableScriptTest()  # None type in outputs
     def test_googlenet(self):
         x = Variable(torch.randn(BATCH_SIZE, 3, 224, 224).fill_(1.0))

Diff for: test/onnx/test_pytorch_onnx_onnxruntime.py

@@ -5998,6 +5998,20 @@ def forward(self, input):
         x = torch.randn(6, 4, 3, 3)
         self.run_test(FakeQuantizePerTensorModel(), (x))
 
+    @skipIfUnsupportedMinOpsetVersion(13)
+    def test_fake_quantize_per_channel(self):
+        class FakeQuantizePerChannelModel(torch.nn.Module):
+            def forward(self, input):
+                amax = torch.ones(4)
+                scale = amax / 127.
+                zero_point = torch.zeros_like(amax, dtype=torch.long)
+                # Quantize twice to test differnet branches
+                y = torch.fake_quantize_per_channel_affine(input, scale, zero_point, 1, 0, 255)
+                return torch.fake_quantize_per_channel_affine(y, scale, zero_point, 1, -128, 127)
+
+        x = torch.randn(6, 4, 3, 3)
+        self.run_test(FakeQuantizePerChannelModel(), (x))
+
     def test_batchnorm_training(self):
         class MyModule(torch.nn.Module):
             def __init__(self):

Diff for: torch/onnx/symbolic_opset13.py

@@ -121,6 +121,21 @@ def where(g, condition, self=None, other=None, _outputs=None):
         return sym_help._unbind_helper(g, condition, g.op("Constant", value_t=torch.tensor(1)), _outputs)
     return g.op("Where", condition, self, other)
 
+@parse_args('v', 'v', 'v', 'i', 'i', 'i')
+def fake_quantize_per_channel_affine(g, inputs, scale, zero_point, axis, quant_min=-128, quant_max=127):
+    if quant_min not in [0, -128] or quant_max not in [127, 255]:
+        raise RuntimeError(
+            "ONNX defines [0, 255] for quint8 and [-128, 127] for qint8, got [{}, {}]".format(quant_min, quant_max))
+
+    # ONNX defines zero_point to be int8 or uint8
+    if quant_min == 0:
+        zero_point = g.op("Cast", zero_point, to_i=sym_help.cast_pytorch_to_onnx['Byte'])
+    else:
+        zero_point = g.op("Cast", zero_point, to_i=sym_help.cast_pytorch_to_onnx['Char'])
+    return g.op(
+        "DequantizeLinear",
+        g.op("QuantizeLinear", inputs, scale, zero_point, axis_i=axis),
+        scale, zero_point, axis_i=axis)
 
 def _reduce_op_symbolic(onnx_op_name):
     def symbolic(g, self, dim=None, keepdim=None):