create LxmertModelIntegrationTest Pytorch
# What does this PR do?

this pr fix issue #9951 as it implements an integration test for  LXMERT

@LysandreJik

Hello! It seems this is simply passing the test?<|||||>> Hello! It seems this is simply passing the test?

I made it to claim the issue and work on it. <|||||>@LysandreJik  Lxmert requires `visual_feats` and `visual_pos` could I change the `model.config.visual_feats_dim` to a smaller value like 5 or 10. 
Edit: or we could use a `seed` and generate a random tensor with original `visual_feats_dim`<|||||>@LysandreJik  in this I used `np.random.seed` to fix the `visual_feats` and `visual_pos`,  otherwise we could load lxmertmodel by lxmertconfig while changing `visual_feat_dim` to something manageable. 

What do you suggest?<|||||>@LysandreJik  is there something I can help with here?<|||||>> Could you try to replace the `np.random.rand` by the `ids_tensor`
well the `visual_feats` is torch.float will the `ids_tensor` returns int32.


@LysandreJik  I think another alternative, is to make the `model.config.visual_feat_dim` smaller then we can have a fixed `visual_feats` 


what do you think?
<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>@LysandreJik  I did raise an issue to have a context manager that will fix a seed #10143, do you think it will be usefull here?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hello @sadakmed, sorry for taking a while to merge this. Do you mind rebasing on `master`, and running `make fixup` at the root of your clone? There's an issue with the code quality.

Will merge this right after.<|||||>Thanks a lot for your contribution @sadakmed!