ONNX compatibility

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.