Implement `embed` and `time_embed` for the `BaseTSFMSolver`

A first attempt at simplifying the code for models has been to code a new parent `BaseTSFMSolver` class in which most of the logics for forecasting is located (such that each new model would just need to inherit + implement `batch_forecast` and a model loader).

The next step would be to do the same for `embed` and `time_embed`, see discussion #31
That means having `batch_embed` and `batch_time_embed` (signature defined in `BaseTSFMSolver` with tensor shapes specified in the docs, actual implementation deferred per model).

Once this is done, a full pass on the whole set of available models should be done to make sure they stick (if it makes sense at least) to that behaviour.
Also, Chronos vs Chronos-v2 is a mess at the time, this will need to be improved at some point.

Start with the BaseTSFMSolver definition and discussion #31 to confirm the batch_embed and batch_time_embed signatures and documented tensor shapes. Review the available model implementations, add the deferred per-model behavior, and check that the models consistently follow the new embedding interface where applicable.

Written by the indexing model from the issue text.