Do not use GPU when importing transformers
This PR addresses the issue #1507 and the main point is to **not** use GPU already at import time of transformers.

This was caused by the use of class variable `dummy_inputs` that initializes TF by using `tf.constant`. This can be prevented simply by makeing `dummy inputs` a local variable since is not used anywhere else but in `__init__`.

commit messages:
* Make dummy inputs a local variable in TFPreTrainedModel.
Well this is actually used in other places, in `modeling_tf_pytorch_utils` for instance, and designed to be overriden by model class-specific inputs, for instance in `modeling_xlm` (hence all the CircleCI errors).

Maybe making this a python property instead of an attribute would solve your issue as well?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1735?src=pr&el=h1) Report
> Merging [#1735](https://codecov.io/gh/huggingface/transformers/pull/1735?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ba973342e3315471a9f44e7465cd245d7bcc5ea2?src=pr&el=desc) will **increase** coverage by `1.39%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1735/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1735?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1735      +/-   ##
==========================================
+ Coverage   82.56%   83.95%   +1.39%     
==========================================
  Files          94       94              
  Lines       13951    13952       +1     
==========================================
+ Hits        11519    11714     +195     
+ Misses       2432     2238     -194
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1735?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3V0aWxzLnB5) | `92.45% <100%> (+0.04%)` | :arrow_up: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `81.69% <0%> (+1.35%)` | :arrow_up: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `96.36% <0%> (+2.27%)` | :arrow_up: |
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `73.28% <0%> (+2.46%)` | :arrow_up: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `71.08% <0%> (+12.65%)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `96.8% <0%> (+17.02%)` | :arrow_up: |
| [transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1735/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3B5dG9yY2hfdXRpbHMucHk=) | `92.95% <0%> (+83.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1735?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/1735?src=pr&el=footer). Last update [ba97334...124409d](https://codecov.io/gh/huggingface/transformers/pull/1735?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>When will this pull request be merged and released?<|||||>Indeed, LGTM, let's merge