Revamp a few ScratchAllocator classes in cudnn_rnn_ops

Revamp a few ScratchAllocator classes in cudnn_rnn_ops

Prepare for RNN autotune.
* The scratch allocator classes are renamed s.t. they're named by the duration of memory allocated.
  * CudnnReservespaceAllocator ==> CudnnRnnAllocatorInOutput.
  * CudnnWorkspaceAllocator ==> CudnnRnnAllocatorInTemp

* The old CudnnWorkspaceAllocator (new CudnnRnnAllocatorInTemp) is made a template s.t. it works with different tensor dtypes, which is used later in autotune, during which both workspace (uint8) and reserve space (input_dtype) are temp-allocated.

* Change CudnnModelShapes  ==> CudnnRnnModelShapes

PiperOrigin-RevId: 192018334