Generate: add missing `**model_kwargs` in sample tests
# What does this PR do?

One call of the `sample`-related tests was missing `**model_kwargs`, which MAY explain the random failures we were seeing. 

I've run all `test_sample_generate_dict_output` tests 100x with no failures. Before this change, it was failing once every ~10 calls of `py.test tests/ -k test_sample_generate_dict_output`.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh when the attention mask is not passed, it is inferred from the input ([here](https://github.com/huggingface/transformers/blob/e54a1b49aa6268c484625c6374f952f318914743/src/transformers/generation_utils.py#L490)).

Depending on the pad and eos tokens, the mask inferred from the random input tokens may have issues. This automatic attention mask is a big source of issues in general, especially for the tests :( 

Passing the mask explicitly is always preferred!