Add argument "cache_dir" for transformers.onnx
# What does this PR do?


Add argument "cache_dir" for transformer.onnx ,to enables the user to select the cache path when exporting large ONNX model. Because the default cache directory may run out of space.


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_The documentation is not available anymore as the PR was closed or merged._<|||||>Looking at the CI, it seems you need to format your files @happyXia - can you please run:

```
make style && make quality
```

and commit / push the formatted files?<|||||>> Looking at the CI, it seems you need to format your files @happyXia - can you please run:
> 
> ```
> make style && make quality
> ```
> 
> and commit / push the formatted files?

Sure, I have reformated and pushed the flawed files.
<|||||>I checked that the slow tests pass with

```
RUN_SLOW=1 python -m pytest -x tests/onnx/test_onnx_v2.py
```

so this all looks good to me - merging now. Thank you @happyXia 🚀 !