Expected behavior in sklearn backend

[DrAnaximandre]

In the sklearn backend:

• The following code Expe.fit([data_0],[]) (providing a validation data that is an empty list) does not return an error, but does not returns a result either. So far, what the user should do (if no validation data is avialable) is Expe.fit([data_0],[None]). Proposed fix: assert that the length of data and data val is the same in train.

• The following code Expe.fit([data_0,data_1],[data_val]) only fits on data_0 (and produces a score of length 1) while Expe.fit([data_0,data_1],[data_val,None]) fits both data (and produces a score of length 2). Proposed fix: same as before

[tboquet]

It should be possible to extend the list so the many to one example works.