Continual-inference

Added
- `co.Linear` module.
- Improved repr in `co.Lambda`.
- Option to skip unsqueeze in `co.Lambda.forward_step`.

Changed
- Renamed `co.Parallel` to `co.BroadcastReduce`.

Added
- `co.Broadcast` module.
- new `co.Parallel` module.
- `co.Reduce` module.
- Automatic inference of `co.Broadcast.num_streams` in `co.Sequential`.

Added
- `co.Lambda` module.
- `co.Add` module.
- `co.Multiply` module.
- `co.Unity` module.
- `co.Conditional` module.

Fixed
- Bug in `forward_stepping`.
- Bug in `clean_state`.

Fixed
- Bugs in `forward_step(s)` with `update_state=False`.

Changed
- `forward_steps` interface to always include `pad_end` argument.
- Name of "interface.py" to "module.py".
- Implementations of `forward_step(s)` to be consolidated in CoModule.

Removed
- `Padded` interface.

Added
- Independent state_dict and load_state_dict functions.
- Added nonempty check for aggregation functions in Parallel.
- `update_state` argument to all `forward_step(s)` methods.
- Additional tests for edge-cases

Changed
- Changed default pad_end value to False.

Fixed
- Continual interface and conversion to support both class and module.
- Replicate padding in `co._ConvNd`