[MRG+1] Catch cases for different class size in MLPClassifier with warm start (#7976) (#8035)

* added test that fails

* generate standard value error for different class size

* moved num_classes one class down

* fixed over-indented lines

* standard error occurs a layer up.

* created a different label comparison for warm_start

* spaces around multiplication sign.

* reworded error and added another edge case.

* fixed pep8 violation

* make test shorter

* updated ignore warning