RNN cell with list of symbol as input

[Godricly]

@piiswrong
when using list of symbol as input, the layer is assumed as TNC order, While the default layout input is NTC. It is not properly parsed for list input case.

assert len(inputs) == length
inputs = [symbol.expand_dims(i, axis=0) for i in inputs]
inputs = symbol.Concat(inputs, dim=0)
if begin_state is None:
begin_state = self.begin_state()

Also I think the Concat is not right here. which should be:

inputs = symbol.Concat(*inputs, dim=0)

[Godricly]

problem fixed with #5166