[Add] SLURP models and examples
The PR adds the new SOTA model we have for intent classification and slot filling for spoken language understanding.

- [x] Copy transformer modules from `nemo.collections.nlp.modules.common` to `nemo.collections.asr.modules`
- [x] Add models, loss and utils to nemo.collection.asr
- [x] Add example scripts for data downloading and processing 
- [x] Add example scripts for training and evaluation 
- [x] Refactor model to be independent of EncDecCTCBPE
- [x] Refactor to use built-in SmoothedCrossEntropyLoss
- [x] Write README for example 
- [x] Upload model checkpoints to NGC and create model cards
- [x] Update Docs and list_available_models()
- [ ] Create tutorial. // Plan to add in later PR

The NGC model card is [here](https://ngc.nvidia.com/models/nvstaging:nemo:slu_conformer_transformer_large_slurp)
The HF model card is [here](https://huggingface.co/nvidia/slu_conformer_transformer_large_slurp)