Data-info fields:
* Rename "single_input_shape" to "features_shape".
* Move deprecation warnings to `combine` classmethod as the data-
  info fields are never instantiated.
* Make these deprecation warnings more verbose.
* On the side, remove some redundant-with-parent type hints.

Models:
* Have "features_shape" verification happen for both regression
  and classification variants of `SklearnSGDModel`.
* Remove requirement for "n_samples" in `TensorflowModel`.