Skip pt/flax equivalence tests in pytorch `bigbird` test file
# What does this PR do?

#21023 fixed random attention issue in Flax bigbird model, and skipped the pt/flax equivalence tests in flax bigbird test file with
```txt
reason="Current Pytorch implementation has bug with random attention -> it always uses it not matter if we are in eval/train mode"
``` 
We need to skip the pt/flax equivalence tests in **pytorch** bigbird test file too.

Currently on `main`, the tests fail

https://app.circleci.com/pipelines/github/huggingface/transformers/63217/workflows/5d512271-f535-44be-a2ec-b95024f8f165/jobs/780069
cc @Bearnardd<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Good catch @ydshieh! I will fix pytorch's big bird implementation today/tomorrow.<|||||>@Bearnardd Thank you 🤗 . Luckily, there is no `TFBigBirdModel` 😆 