Add CircleCI config to run TPU tests.
Author: zcain117
Pull request: yes
Labels: name: model card

For every incoming commit, this PR will create a Docker image containing the commit's latest code and will run that Docker image on Google Kubernetes Engine on a TPU.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6158?src=pr&el=h1) Report
> Merging [#6158](https://codecov.io/gh/huggingface/transformers/pull/6158?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ec0267475c16a1913e64cb4f81fd54d153e3d815&el=desc) will **decrease** coverage by `1.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6158/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6158      +/-   ##
==========================================
- Coverage   79.38%   78.36%   -1.03%     
==========================================
  Files         146      146              
  Lines       26454    26454              
==========================================
- Hits        21001    20730     -271     
- Misses       5453     5724     +271     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+0.75%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <0.00%> (+1.16%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `95.23% <0.00%> (+10.71%)` | :arrow_up: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `84.09% <0.00%> (+12.87%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.80% <0.00%> (+25.66%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `95.32% <0.00%> (+29.90%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6158?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/6158?src=pr&el=footer). Last update [ec02674...a662ce9](https://codecov.io/gh/huggingface/transformers/pull/6158?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


`check_code_quality` failed with:

```
    torch from https://files.pythonhosted.org/packages/38/53/914885a93a44b96c0dd1c36f36ff10afe341f091230aad68f7228d61db1e/torch-1.6.0-cp36-cp36m-manylinux1_x86_64.whl#sha256=7669f4d923b5758e28b521ea749c795ed67ff24b45ba20296bc8cff706d08df8 (from transformers==3.0.2):
        Expected sha256 7669f4d923b5758e28b521ea749c795ed67ff24b45ba20296bc8cff706d08df8
             Got        36bbf4ab202de410d764b9156f3925b7d7037ad046f20690e576725a3826a2ac
```

I don't see how this latest commit could have caused this. I'll retry later

See https://github.com/huggingface/transformers/pull/6219