[ASR] Conformer global tokens in local attention
# What does this PR do ?

Adds Longformer-style attention for Conformer encoder (limited context + few tokens with full context attention). Unlike with just limited context attention, when using global tokens model needs to be fine-tuned with this approach to get good results. After fine-tuning results are improved particularly on long audio (even if it's much longer than training data). When using the default Fast Conformer configs with this attention, inference on audio above 1 hour is supported.

**Collection**: ASR

# Changelog 
- Add option to use global tokens
- Adds corresponding test
- Configs for training Conformer with this attention and mention in docs

# Usage

Add the following parameters to the Conformer encoder in config:



```  
    self_attention_model: rel_pos_local_attn # longformer-style attention (sliding window + global tokens)
    global_tokens: 1 # number of tokens that attend and are attended to by all tokens
    global_attn_separate: false # whether global tokens should use separate q,k,v layers
```

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

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# Additional Information
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