text: Title: [ENH] in `TimesFMForecaster`, `context_len` and `horizon_len` should be set automatically per default
  Body: `TimesFMForecaster` should set `context_len` and `horizon` len to a reasonable value, automatically per default.
  
  The `horizon_len` can be obtained from `fh`, but it makes sense to give the user the option to override.
  
  The `context_len` can also be set to an automatic choice, in the default. Again, giving the user the option to override makes sense.
  
  FYI @geetu040.
  
  As a recipe to change this, there needs to be an internal `self._context_len` and `self._horizon_len` which is determined by the settings.