Continual-inference

Added
- ONNX compatibility to all library modules!
- `co.onnx.export` function mirroring `torch.onnx.export`.
- purely functional `_forward_step` function to all modules.
- `_state_shape` and `_dynamic_state_inds` properties to modules.
- Add about info to package root.

Changed
- Change call_mode internally from enum to tensor.
- Change internal state_indexes to tensors.
- Change stride to tuple.
- Change padding to tuple.

 Fixed
- Fix assertion bug in co.Lambda.

 Removed
- TensorPlaceholder in favour of `None`.

Added
- Missing part on Continual Transformers in README.

Removed
- Conv cpp impl.

Added
- Citations for Continual Inference lib paper.
- Docs.
- Automatic conversion for RNN modules.
- Continual Transformer modules, including:
- `RecyclingPositionalEncoding`
- `RetroactiveMultiheadAttention`
- `SingleOutputMultiheadAttention`
- `SingleOutputTransformerEncoderLayer`
- `RetroactiveTransformerEncoderLayer`
- `TransformerEncoderLayerFactory`
- `TransformerEncoder`

Added
- "lagging" option for shrink in `co.Delay` and `co.Residual`.
- `co.RNN`.
- `co.LSTM`.
- `co.GRU`.

Changed
- `phantom_padding` renamed to `residual_shrink`.

Fixed
- Missing cpp file in package.

Added
- CoConv step impl in C++.