Fix the __getattr__ method in BatchEncoding
Fix the issue where the `__getattr__` method in `BatchEncoding` was raising a `KeyError` instead of an `AttributeError` when the attribute was accessed with `getattr()`.

Example:
```
from transformers import BertTokenizer
tokenizer = BertTokenizer.from_pretrained("bert-base-cased")
features = tokenizer.encode_plus("Hello here")
getattr(features, "attr", False)
```

Previous output:
```
/home/jplu/transformers/src/transformers/tokenization_utils.py:204 __getattr__
        return self.data[item]
    KeyError: 'attr'
```

New output:
```
False
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=h1) Report
> Merging [#4772](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5856999a9f2926923f037ecd8d27b8058bcf9dae&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4772/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4772      +/-   ##
==========================================
- Coverage   77.98%   77.96%   -0.03%     
==========================================
  Files         123      123              
  Lines       20436    20437       +1     
==========================================
- Hits        15938    15933       -5     
- Misses       4498     4504       +6     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.47% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/4772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcGkucHk=) | `93.06% <0.00%> (-4.96%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.50% <0.00%> (-0.17%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=footer). Last update [5856999...fa0abc6](https://codecov.io/gh/huggingface/transformers/pull/4772?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
