DOC: Make `import torch` explicit for "Quick tour TF 2.0" example
I tried to run the Quick Tour example with only the `tensorflow` and the `transformers` imports (as shown literally in the code snippet), and _obviously_ (in hint sight) this fails with:

```
pytorch_model = BertForSequenceClassification.from_pretrained('./models/', from_tf=True)
NameError: name 'BertForSequenceClassification' is not defined
```

The trivial fix was to add `import torch` to the snippet.

When running all examples in sequence, this is not an issue, but I
was running the `tensorflow 2` example in a separate project.

Adding this line may avoid this confusion for the next newcomer :-)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=h1) Report
> Merging [#4537](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a34a9896ac2a4a33ff9cd805c76eed914c8d8965&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4537/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4537      +/-   ##
==========================================
- Coverage   77.87%   77.86%   -0.01%     
==========================================
  Files         123      123              
  Lines       20566    20566              
==========================================
- Hits        16016    16014       -2     
- Misses       4550     4552       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4537/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4537/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4537/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=continue).
> **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/4537?src=pr&el=footer). Last update [a34a989...fc189e7](https://codecov.io/gh/huggingface/transformers/pull/4537?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
