[Op] cuDNN RNN Symbol (#2795)

* - first commit

* - removed unnecssary commented out code
- fixed error in output shape inference

* - some renaming
- added cudnn destructors

* - added dropout

* - major refactor
- completed forward evaluation

* - added parameter size test
- fixed bug where cudnnGetRNNParamsSize needs to be called after cudnnSetRNNDescriptor

* - checks for contiguous input tensors
- more consistent param names
- removed 'batch_first' option for now. Might add it later again

* - fixed input names

* - added backward method

* - small fix for in/out names

* - fixed bug: parameters can't have underscore

* - fixed off-by-two error in weight shape inference for bidirectional net
- moved calculated param to cudnn_rnn-inl.h

* - added option to control num outputs

* - removed lint

* - correct handling of backward dependencies

* - fix lint

* - first commit

* - removed unnecssary commented out code
- fixed error in output shape inference

* - some renaming
- added cudnn destructors

* - added dropout

* - major refactor
- completed forward evaluation

* - added parameter size test
- fixed bug where cudnnGetRNNParamsSize needs to be called after cudnnSetRNNDescriptor

* - checks for contiguous input tensors
- more consistent param names
- removed 'batch_first' option for now. Might add it later again

* - fixed input names

* - added backward method

* - small fix for in/out names

* - fixed bug: parameters can't have underscore

* - fixed off-by-two error in weight shape inference for bidirectional net
- moved calculated param to cudnn_rnn-inl.h

* - added option to control num outputs

* - removed lint

* - correct handling of backward dependencies

* - fix lint

* - fix type narrowing bug

* - fixed incorrect dropout parameter
- added dropout states
- fixed incorrect handling of variable outputs

* - fix incorrect cell state forward handling

* - fixed lint by replacing unsigned long long with uint64_t

Author: sbodenstein