[TFGPT2] - Fix flaky past_key_values test
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR attempts to fix the flaky TFGPT2 test: [tests/test_modeling_tf_gpt2.py::TFGPT2ModelTest::test_gpt2_model_past_large_inputs](https://app.circleci.com/pipelines/github/huggingface/transformers/18086/workflows/2c889716-285d-489f-9c1b-03c99155ea37/jobs/145873)

To be honest, I'm not really sure what is/was going on there. I don't see an obvious bug in any of the test and `TFGPT2` wasn't changed for a long time -> so not sure what's going on. Also before doing the changes in the PR the test failed 1/20 times in my bash loop. 

The only change in this PR is to change the batch_size from 13 to 1 as it's done in other TF tests as well (see: https://github.com/huggingface/transformers/blob/a400fe8931cce276df74c7c7a5ee4dd28b5674ec/tests/test_modeling_tf_t5.py#L203). 
=> so I think the test should have passed previously as well (there should be no difference between batch_size 1 and 13 ...)

After the change, I ran the test 60 times in a loop and it never failed - we should still keep an eye on it though.


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