title: dropout(): argument 'input' (position 1) must be Tensor, not tuple when using XLNet with HuggingfCE
body: <p>I get an error saying that the input should be of type Tensor, not tuple. I do not know how to work around this problem, as I am already implementing the return_dict=False method as stated in the migration plan.</p>
  <p>My model is as follows:</p>
  <pre class="lang-py prettyprint-override"><code>class XLNetClassifier(torch.nn.Module):
      def __init__(self, dropout_rate=0.1):
          super(XLNetClassifier, self).__init__()
          self.XLNet = XLNetModel.from_pretrained('xlnet-base-cased', return_dict=False)
          self.d1 = torch.nn.Dropout(dropout_rate)
          self.l1 = torch.nn.Linear(768, 64)
          self.bn1 = torch.nn.LayerNorm(64)
          self.d2 = torch.nn.Dropout(dropout_rate)
          self.l2 = torch.nn.Linear(64, 3)
          
      def forward(self, input_ids, attention_mask):
          x = self.XLNet(input_ids=input_ids, attention_masks = attention_mask)
          x = self.d1(x)
          x = self.l1(x)
          x = self.bn1(x)
          x = torch.nn.Tanh()(x)
          x = self.d2(x)
          x = self.l2(x)
          
          return x
  </code></pre>
  <p>The error occurs when calling the dropout.</p>