Add support to change Multi task model prompt
# What does this PR do ?

Adds the ability to change prompt format for multi task models

**Collection**: [ASR]

# Changelog 
- Add function change_prompt().
- Add tests for changing prompt format of function

# Usage

```python
from nemo.collections.asr.models import ASRModel
from nemo.collections.common.prompts.canary import CanaryPromptFormatter

model = ASRModel.from_pretrained("nvidia/canary-1b")

class CanaryPromptFormatterSubclass(CanaryPromptFormatter):
    NAME = "canary2"

prompt_defaults = asr_model.prompt.get_default_dialog_slots()
user_prompt = prompt_defaults[0]
slots = user_prompt['slots']
slots['source_lang'] = 'en'
slots['target_lang'] = 'en'
slots['task'] = 'asr'
slots['pnc'] = 'no'
model.change_prompt(prompt_format='canary2', prompt_defaults=prompt_defaults)
```

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