Add `qas_id` to SquadResult and SquadExample
I'm in the process of adding a `run_tf_squad.py` script, per https://github.com/huggingface/transformers/issues/3685.

This PR:

- Fixes a buggy variable name: `all_example_indices` actually refers to feature indices, so I've renamed it to `all_feature_indices`. This can be verified by adding a breakpoint at https://github.com/huggingface/transformers/blob/master/src/transformers/data/processors/squad.py#L347 and running
```python
print(len(set(all_example_index))) # 12272
print(len(features)) # 12272
print(len(set([f.example_index for f in features]))) # 11873
```
This is because an `Example` refers to a Question + possibly several Answers.
A `Feature` refers to a Question + one Answer. There are 12272 features, but only 11873 examples in the SQuADv2 dataset.

- Adds two attributes to the TensorFlow SQuAD dataset: `feature_index` and `qas_id`. `feature_index` has the same function as it does in PyTorch, but it is now possible to retrieve through the tf.data API. `qas_id` is the ID of an example, and matches [the JSON here](https://rajpurkar.github.io/SQuAD-explorer/dataset/dev-v2.0.json).

These two features enable a TensorFlow SQuAD validation script. I have it up and running and will include it in a later PR, as support for a native `TFAlbertForQuestionAnswering` is required first.


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=h1) Report
> Merging [#3745](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/700ccf6e35616fcbee59de81edd60cec9e14fb6b&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `14.28%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3745/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3745   +/-   ##
=======================================
  Coverage   78.26%   78.26%           
=======================================
  Files         106      106           
  Lines       17928    17931    +3     
=======================================
+ Hits        14031    14034    +3     
  Misses       3897     3897           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/3745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.61% <14.28%> (-0.28%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.22% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.12% <0.00%> (+0.32%)` | :arrow_up: |

------

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> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=footer). Last update [700ccf6...ce09bce](https://codecov.io/gh/huggingface/transformers/pull/3745?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Bumping this. @LysandreJik Any thoughts?