[Quant][FX] Add backend config for onednn backend and fuse Linear-LeakyReLU (#88665)
**Summary**
Add backend config for onednn backend so that it can support more post op fusion for int8 inference. First `Linear - LeakyReLU` fusion is implemented based on previous PRs.
**Test plan**
python test_quantization.py TestFuseFx
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88665
Approved by: https://github.com/jgong5, https://github.com/jerryzh168
