Add integration tests lm generate torch tf
Add integration tests for all LM models that are able to generate language.

- All integration tests use `do_sample=False` (greedy) generation and verify that TF 2.0 and PT yield the same results. 
- Fixed a small bug with TFXLMModelWithLMHead
Approx. how long do these tests take on a single V100?<|||||>> Approx. how long do these tests take on a single V100?

Don't know how long they take on a V100. On a cpu, all tests combined (7 model tests for PT and 6 model tests for TF) take less than 10min (whereas `test_modeling_tf_xlnet.py`, `test_modeling_xlnet.py` and `test_modeling_transfo_xl.py` combined take ca. 8min)<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=h1) Report
> Merging [#3191](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e03129ad447ad7670fcc6206e5eb27a5435d4d86?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3191/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3191      +/-   ##
==========================================
+ Coverage   78.15%   78.16%   +0.01%     
==========================================
  Files          98       98              
  Lines       16641    16641              
==========================================
+ Hits        13006    13008       +2     
+ Misses       3635     3633       -2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/3191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `90.4% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.56% <0%> (+0.15%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.89% <0%> (+0.19%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=footer). Last update [e03129a...9050ffe](https://codecov.io/gh/huggingface/transformers/pull/3191?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Good to merge for me