fix ONNX support for bloom
Merged on https://github.com/huggingface/transformers/pull/18344

This PR aims to fix ONNX export of bloom. All the following tests are passing:
```
RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -k "bloom"
RUN_SLOW=1 pytest tests/models/bloom/test_modeling_bloom.py
```
_The documentation is not available anymore as the PR was closed or merged._