Conformer local attention
# What does this PR do ?

Sliding window local attention for Conformer.

**Collection**: ASR

# Changelog 
- adds option to use local attention for Conformer
- adds option for overriding/merging parameters into model config when using transcribe/eval scripts or loading model

# Usage

With an eval script can be used like this to switch to local attention with any Conformer checkpoint

```
python speech_to_text_eval.py \
  (...other params...)  \
  ++merge_into_model_config.encoder.att_context_size=[64,64] \
  ++merge_into_model_config.encoder.self_attention_model="rel_pos_local_attn"
```

Or when loading model 

```python
merge_into_model_config = {
  'encoder': dict({
    'self_attention_model': "rel_pos_local_attn",
    'att_context_size': [64, 64]
    }),
}
merge_into_model_config = DictConfig(merge_into_model_config)
asr_model = ASRModel.from_pretrained('stt_en_conformer_ctc_large', map_location='cuda',
  merge_into_model_config=merge_into_model_config)
```

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