[summary] Add annotations to `Recurrent`

[texasmichelle]

The design of Recurrent layer types is unique, but it should still be possible to add annotations. Tackling this separately from #1067, but the principle should be similar.

[texasmichelle]

I expected this to be possible by conforming the associated types for RecurrentLayerCell to DifferentiableTensorProtocol:

public protocol RecurrentLayerCell: Layer
where
  Input == RNNCellInput<TimeStepInput, State>,
  Output == RNNCellOutput<TimeStepOutput, State>
{
  associatedtype TimeStepInput: DifferentiableTensorProtocol
  associatedtype TimeStepOutput: DifferentiableTensorProtocol
  associatedtype State: DifferentiableTensorProtocol
}

This is problematic because LSTMCell defines TimeStepOutput as a custom struct containing both a visible and hidden Tensor<Scalar>.

With the current architecture in its complex form, it is unclear to me how to add annotations without splitting the layer definitions apart.