[squad] make examples and dataset accessible from SquadDataset object
In order to do evaluation on the SQuAD dataset using `squad_evaluate`, the user needs access to both the examples loaded in the dataset and the `TensorDataset` that contains values like `unique_id` and the like that are used in constructing the list of `SquadResult` objects. This PR surfaces the examples and dataset to the user so that they can access it directly.

For example of why access to those is needed, see how evaluation is currently done in `examples/run_squad.py`. The `SquadDataset` object attempts to wrap up some of this functionality, but without access to examples and dataset the evaluation is not possible.
There seem to be some issues with CircleCI right now causing all the integration tests to fail. Please let me know if there is an issue on my end.