Continuation of Issue#352

[nabenabe0928]

The issue is the division between multiclass and continuous.
The quick solution is to filter continuous by checking whether labels cover the integer of [0, C).
However, it causes the following issues:

1. numerical class labels of random integers are not allowed
2. 1-indexed class labels are not allowed
3. rank regression will be mis-recognized as multiclass

To address those issues, we need to enhance input_validator or preprocessing BEFORE instantiating dataset class.
At least, we know that the given task is either classification or regression, so we should use this knowledge.

[ravinkohli]

It has been resolved with #386