Support text types in sklearn predictors

[popescu-v]

Description

Khiops 11 supports Text columns which have a specialized AutoML treatment as oppossed to normal strings(Categorical). Sklearn predictors should also support this type.

Questions/Ideas

• Does pandas and/or numpy have a specialized Text type?
• Option 1: Implement it as a Dataset property
  • Add to the table specification tuple should have an optional field text_columns with the names of the text fields or
  • Add another field to the spec table_text_columns indexed by the table name and whose values are the names of the text columns (I prefer this one)
  • When creating the dictionary the Dataset object will have all the necessary info to add the specified columns as Text
• Option 2: Implement it as a fit parameter
  • As above add table_text_columns but as a fit optional parameter
  • I works but the fact that a column is a Text is part of the description of the dataset
  • This parameter should be feeded to the dictionary creation routine
• Expose Dataset API it will have two init patters:
  • Big Constructor
  • Builder pattern
• The big constructor uses the builder
• And the dict interface should be maintained

[popescu-v]

In pandas >= 1.0.0 there is a StringDType (see https://pandas.pydata.org/docs/reference/api/pandas.StringDtype.html#pandas.StringDtype), but AFAIU it's considered experimental for now.

[popescu-v]

:thumbsup: for Option 1 (IMHO, having a text type column should be an intrinsic property of the dataset, not just a particular usage detail; should we need to reinterpret the text column as categorical in a particular context, we should be able to create a new dataset with a new spec).