remove leftover usage of DUMMY_INPUTS
Author: julian-pani
Pull request: yes

Hey @thomwolf  
This change https://github.com/huggingface/transformers/commit/da26bae61b8c1e741fdc6735d46c61b43f649561#diff-8ddce309e88e8eb5b4d02228fd8881daL28 removed the constant `DUMMY_INPUTS`, but one usage of that constant remains in the code.

So any call to `load_tf2_checkpoint_in_pytorch_model` is currently throwing: `NameError: name 'DUMMY_INPUTS' is not defined`

```
/usr/local/lib/python3.6/dist-packages/transformers/modeling_tf_pytorch_utils.py in load_tf2_checkpoint_in_pytorch_model(pt_model, tf_checkpoint_path, tf_inputs, allow_missing_keys)
    199 
    200     if tf_inputs is None:
--> 201         tf_inputs = tf.constant(DUMMY_INPUTS)
    202 
    203     if tf_inputs is not None:

NameError: name 'DUMMY_INPUTS' is not defined
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1509?src=pr&el=h1) Report
> Merging [#1509](https://codecov.io/gh/huggingface/transformers/pull/1509?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a701c9b32126f1e6974d9fcb3a5c3700527d8559?src=pr&el=desc) will **decrease** coverage by `1.24%`.
> The diff coverage is `6.25%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1509/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1509?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1509      +/-   ##
==========================================
- Coverage   85.98%   84.74%   -1.25%     
==========================================
  Files          91       91              
  Lines       13579    13594      +15     
==========================================
- Hits        11676    11520     -156     
- Misses       1903     2074     +171
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1509?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3B5dG9yY2hfdXRpbHMucHk=) | `9.85% <0%> (-66.91%)` | :arrow_down: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `79.78% <6.66%> (-16.75%)` | :arrow_down: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `65.46% <0%> (-15.11%)` | :arrow_down: |
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `70.87% <0%> (-2.46%)` | :arrow_down: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `93.18% <0%> (-2.28%)` | :arrow_down: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/1509/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `80.4% <0%> (-1.36%)` | :arrow_down: |

------

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Oh nice catch. Let's add a test on `load_tf2_checkpoint_in_pytorch_model` to catch such errors going forward.

Ok, merging, thanks

I was wondering where I should add that test and saw you just did it yourself :) thanks