[Generation Tests] Small speed-up by just generating two tokens
# What does this PR do?

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@LysandreJik @sgugger I originally thought that the PR: https://github.com/huggingface/transformers/commit/c89f1bc92e340600bde526b7ff54ad692b4e48c9 made the PyTorch tests much slower, but after checking the time of `run_tests_torch` in 10+ merges to master after and before this commit, I noticed that the PR didn't really affect the testing time of PyTorch. 

The testing time varies quite a bit, but it seemed on average to be a bit higher after the merged PR, so in this PR I want to reduce  the testing time for generation a bit. The generation length is reduced by one which halves the testing time of all generation tests by 30% without any loss in testing coverage / cases. Generating two tokens is enough => the first token can be generated without `past_key_values`, but the second token has to be generated with `past_key_values` if `use_cache` is enabled and all generation steps following this one can only be the same. So we should always test at least two tokens, but don't really need to test more in general generation tests that apply to all models.

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Just noticed that generation tests are completely irrelevant for the overall testing time...no generation test takes more than 0.5 seconds and 95 % of the generation tests take less than 0.05 seconds