[MNT] Expose PositionalEncoding and TokenPredictor in aptatrans/layers/__init__.py

[Ramrajnagar]

The _encoder.py module defines three public classes: EncoderPredictorConfig, PositionalEncoding, and TokenPredictor. However, layers/init.py only re-exports EncoderPredictorConfig. The other two classes should also be re-exported for API consistency.

[pranavchoudhary-tech]

I'd like to work on this issue! I'll add PositionalEncoding and TokenPredictor to the __all__ list in aptatrans/layers/__init__.py and re-export them to keep the API consistent. Opening a PR shortly.