[squad] make examples and dataset accessible from SquadDataset object
Author: lazovich
Pull request: yes

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.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=h1) Report
> Merging [#6710](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0344428f7955675847ef95ddcb4980236b6f8721?el=desc) will **decrease** coverage by `1.27%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6710/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6710      +/-   ##
==========================================
- Coverage   80.06%   78.78%   -1.28%     
==========================================
  Files         156      156              
  Lines       28386    28391       +5     
==========================================
- Hits        22726    22367     -359     
- Misses       5660     6024     +364     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/datasets/squad.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL3NxdWFkLnB5) | `44.31% <0.00%> (-2.67%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `21.12% <0.00%> (-71.05%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `77.37% <0.00%> (-19.71%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.34% <0.00%> (-1.63%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.71% <0.00%> (-0.76%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `88.05% <0.00%> (+0.55%)` | :arrow_up: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `83.83% <0.00%> (+6.20%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6710/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.22% <0.00%> (+47.80%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=footer). Last update [0344428...20d0228](https://codecov.io/gh/huggingface/transformers/pull/6710?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@sgugger @LysandreJik thanks so much for the comments/suggestions! I have updated the code to include support for the legacy cache format. I had a question on one comment, but if there are any other changes needed please let me know.