

TITLE
[Good first issue] LXMERT PyTorch Integration tests

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The PyTorch implementation of the LXMERT model currently has no integration tests. This is problematic as the behavior can diverge without being noticed.

The [test_modeling_lxmert.py](https://github.com/huggingface/transformers/blob/master/tests/test_modeling_lxmert.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.