remap MODEL_FOR_QUESTION_ANSWERING_MAPPING classes to names auto-generated file
As discussed in https://github.com/huggingface/transformers/issues/10467 currently Trainer loads `models.auto.modeling_auto` which loads **all** modeling files, which is absolutely not needed most of the time. At times it could have unwanted side-effects, loading modeling files that require some 3rd party modules that could be a problem.

Similar to the autogenerated 3rd party version numbers look up dict, this PR autogenerates the names of the classes dict for `MODEL_FOR_QUESTION_ANSWERING_MAPPING` which can then be quickly loaded.

This of course can be extended in the future to generate other structures if need be.

@sgugger, @LysandreJik 

Fixes: https://github.com/huggingface/transformers/issues/10467
