[MISC] Move validation and training parameters to their own components

[blythed]

In order to not create implicit namespaces in the __init__ methods of Component descendants, we should
separate these into their own Component types:

For example:

• train_X
• valid_X

Instead, _Fittable, for example should have a single trainer attribute, which includes these metadata.
Likewise _Validator should include validation: Validator as a single parameter.