pytorch mlp add explicit annotation to fix mypy error

freqtrade/freqai/torch/PyTorchMLPModel.py

@@ -48,7 +48,7 @@ class PyTorchMLPModel(nn.Module):
         self.dropout = nn.Dropout(p=dropout_percent)
 
     def forward(self, x: List[torch.Tensor]) -> torch.Tensor:
-        x = x[0]
+        x: torch.Tensor = x[0]
         x = self.relu(self.input_layer(x))
         x = self.dropout(x)
         x = self.blocks(x)