Fix meta registration for aten._cudnn_rnn

[yanboliang]

Found this issue from weekly running 7k github models. 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'

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[yanboliang]

@pytorchbot merge

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