Fix the self-attention export bug for cache-aware streaming Conformer
# What does this PR do ?

Fixed the self-attention export bug for cache-aware streaming Conformer

# Changelog 
- There was an issue with ONNX conversion of cache-aware streaming Conformer in the self-attention module which raised error for empty caches. Fixed it with wrapping the code by torch script.


**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation