Replace reshape with flatten (#3462)

Current implementation is generating bad graph after onnx conversion. So replacing with flatten like in mobilenetv3 code.
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

torchvision/models/mobilenetv2.py

+import torch
 from torch import nn
 from torch import Tensor
 from .utils import load_state_dict_from_url
@@ -189,8 +190,9 @@ class MobileNetV2(nn.Module):
         # This exists since TorchScript doesn't support inheritance, so the superclass method
         # (this one) needs to have a name other than `forward` that can be accessed in a subclass
         x = self.features(x)
-        # Cannot use "squeeze" as batch-size can be 1 => must use reshape with x.shape[0]
-        x = nn.functional.adaptive_avg_pool2d(x, (1, 1)).reshape(x.shape[0], -1)
+        # Cannot use "squeeze" as batch-size can be 1
+        x = nn.functional.adaptive_avg_pool2d(x, (1, 1))
+        x = torch.flatten(x, 1)
         x = self.classifier(x)
         return x