Fix meta registration for aten._cudnn_rnn (#91333)
Found this issue from [weekly running 7k github models](https://github.com/pytorch/torchdynamo/issues/1884). This caused regression on pass rate, there are 25 models failed due to this issue.
The reason is argument ```cx``` of ```aten._cudnn_rnn``` can be ```None```, but it doesn't handle well in meta registration, so throws the following error:
```
Traceback (most recent call last):
File "/scratch/ybliang/work/repos/pytorch/torch/_dynamo/utils.py", line 1059, in run_node
return nnmodule(*args, **kwargs)
File "/scratch/ybliang/work/repos/pytorch/torch/nn/modules/module.py", line 1482, in _call_impl
return forward_call(*args, **kwargs)
File "/scratch/ybliang/work/repos/pytorch/torch/nn/modules/rnn.py", line 477, in forward
result = _VF.rnn_tanh(input, hx, self._flat_weights, self.bias, self.num_layers,
File "/scratch/ybliang/work/repos/pytorch/torch/_subclasses/fake_tensor.py", line 916, in __torch_dispatch__
r = func(*args, **kwargs)
File "/scratch/ybliang/work/repos/pytorch/torch/_ops.py", line 284, in __call__
return self._op(*args, **kwargs or {})
File "/scratch/ybliang/work/repos/pytorch/torch/_meta_registrations.py", line 2108, in _cudnn_rnn
cy = cx.new_empty(0 if cx is None else cell_shape)
AttributeError: 'NoneType' object has no attribute 'new_empty'
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91333
Approved by: https://github.com/ezyang
