[GPT-J] Use the `float16` checkpoints in integration tests
This PR switches GPTJ checkpoints in the integration tests to fp16 to test if they're able to run on our daily CI.
At the moment, fp32 checkpoints are timing out either during model downloads or initialization:
```
600.01s call     tests/test_modeling_gptj.py::GPTJModelTest::test_batch_generation
600.00s call     tests/test_modeling_gptj.py::GPTJModelLanguageGenerationTest::test_lm_generate_gptj
600.00s call     tests/test_modeling_gptj.py::GPTJModelLanguageGenerationTest::test_gptj_sample_max_time
600.00s call     tests/test_modeling_gptj.py::GPTJModelTest::test_model_from_pretrained
600.00s call     tests/test_modeling_gptj.py::GPTJModelLanguageGenerationTest::test_gptj_sample
```

Note that this doesn't guarantee reproducibility of the old tests (some tokens may be different), but it could help with **caching the models** on the runner to avoid timeouts.

:warning: The tests should be revisited once more, once https://github.com/huggingface/transformers/pull/13466 is merged
`test_gptj_sample()` and `test_gptj_sample_max_time()` were disabled due to GPU OOM during more than one call to `.generate()`