Improve dtype handling in 'SklearnSGDModel'.

- Scikit-Learn 1.3.0 introduced the possibility to use any dtype for
  SGD-based models' weights.
- As a consequence, this commit introduces the optional 'dtype' argument
  to 'SklearnSGDModel.__init__' and '.from_parameters' methods, which is
  only used with `sickit-learn >=1.3` and an un-initialized model.
- Coherently (and for all scikit-learn versions), additional type/dtype
  verification has been implemented under the hood of `_unpack_batch`.