feat(ppca): Add transform and fit_transform methods for sklearn compatibility

[joon0390]

mplement the transform and fit_transform methods to make the PPCA
class fully compatible with the scikit-learn estimator interface.

• The transform method returns the posterior mean of the latent
  variables, which corresponds to the dimension-reduced data.
• This allows the PPCA model to be seamlessly integrated into
  scikit-learn tools like Pipeline