Fix inception v3 input transform for trace & onnx (#621)

* Fix inception v3 input transform for trace & onnx

* Input transform are in-place updates, which produce issues for tracing
and exporting to onnx.

* nit

torchvision/models/inception.py

@@ -70,10 +70,10 @@ def __init__(self, num_classes=1000, aux_logits=True, transform_input=False):
 
     def forward(self, x):
         if self.transform_input:
-            x = x.clone()
-            x[:, 0] = x[:, 0] * (0.229 / 0.5) + (0.485 - 0.5) / 0.5
-            x[:, 1] = x[:, 1] * (0.224 / 0.5) + (0.456 - 0.5) / 0.5
-            x[:, 2] = x[:, 2] * (0.225 / 0.5) + (0.406 - 0.5) / 0.5
+            x_ch0 = torch.unsqueeze(x[:, 0], 1) * (0.229 / 0.5) + (0.485 - 0.5) / 0.5
+            x_ch1 = torch.unsqueeze(x[:, 1], 1) * (0.224 / 0.5) + (0.456 - 0.5) / 0.5
+            x_ch2 = torch.unsqueeze(x[:, 2], 1) * (0.225 / 0.5) + (0.406 - 0.5) / 0.5
+            x = torch.cat((x_ch0, x_ch1, x_ch2), 1)
         # 299 x 299 x 3
         x = self.Conv2d_1a_3x3(x)
         # 149 x 149 x 32