Extend `nested_XXX` functions to mappings/dicts.
Extended `nested_XXX` trainer pt utility functions to work with mappings (dict, OrderedDict, etc.)

Some classes that are modeling the models' outputs, inherit from `ModelOutput` which at turn is an `OrderedDict`. Currently, when applying these `nested_XXX` functions to these models ouputs the code fails with an error.

Extending this nested utilities to work with dictionaries fix this issue.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you for your feedback @sgugger . 
Suggestions and style applied!<|||||>Thanks a lot!