Fix stride default value None in torch.nn.functional.avg_pool (#4984)

* fix unordered dictionary problem for python version 3.5

* modify style

* default value of stride in torch.nn.functional.avg_pool is None

* delete prev modifications

* add testcase for nn.functional.avg_pool2d

python/tvm/relay/frontend/pytorch.py

@@ -470,7 +470,10 @@ def _avg_pool2d():
         data = inputs[0]
 
         pool_size = _infer_shape(inputs[1])
-        strides = _infer_shape(inputs[2])
+        if inputs[2]:
+            strides = _infer_shape(inputs[2])
+        else:
+            strides = pool_size
         padding = _infer_shape(inputs[3])
 
         ceil_mode = int(inputs[4])

tests/python/frontend/pytorch/test_forward.py

@@ -375,8 +375,13 @@ def test_forward_avgpool():
         def forward(self, *args):
             return torch.nn.AvgPool2d(kernel_size=[10, 10])(args[0])
 
+    class AvgPool2D2(Module):
+        def forward(self, *args):
+            return torch.nn.functional.avg_pool2d(args[0], kernel_size=[10, 10])
+
     input_data = torch.rand(input_shape).float()
     verify_model(AvgPool2D1().float().eval(), input_data=input_data)
+    verify_model(AvgPool2D2().float().eval(), input_data=input_data)
 
 def test_forward_hardtanh():
     torch.set_grad_enabled(False)