[ExecuTorch] Cannot Convert Empty Model

This toy model fails in ExecuTorch
```
class IndexModel(torch.nn.Module):
def forward(self, x):
if len(shape) == 2:
return x[:, :]
elif len(shape) == 4:
return x[:]
```
It contains only a no op, so ExecuTorch would simply return its input

However, in Core ML input and output must have different names