[Good first issue] BERT Generation PyTorch Integration tests
The PyTorch implementation of the BERT for generation model currently has no integration tests. This is problematic as the behavior can diverge without being noticed.

The [test_modeling_bert_generation.py](https://github.com/huggingface/transformers/blob/master/tests/test_modeling_bert_generation.py) file should be updated to include integration testing.

An example of a good modeling integration test is visible in the [test_modeling_bert.py#L552-L565](https://github.com/huggingface/transformers/blob/master/tests/test_modeling_bert.py#L552-L565) file:

https://github.com/huggingface/transformers/blob/1809de5165804666ba6c6a02a9d177f6683869cc/tests/test_modeling_bert.py#L552-L565

Some additional tips:
- The test must be marked as slow using the `@slow` decorator, so as to be run *daily*, and not on every commit of every branch/pull request of this repository.
- The test must be decorated with the `@require_torch` decorator so as to only be run in environments using PyTorch.
- A single test is necessary. If you feel like implementing multiple of these, then sharing the same checkpoint would be ideal so as to reduce download time.
@LysandreJik  I think this should be closed by now!<|||||>You're correct! Thanks again @sadakmed!