added conformer mandarin model.
Signed-off-by: Vahid <vnoroozi@nvidia.com>

# What does this PR do ?

Adding the checkpoint of a Mandarin ASR model trained with Conformer-Transducer to the pretrained model's list. It is trained on AIShell-2 dataset.

# Changelog 
Added a Mandarin ASR model trained with Conformer-Transducer on AIShell-2.

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