Support Transcribing a Manifest File in `transcribe()` method
# What does this PR do ?

This PR enables transcribing an already existing manifest file instead of providing a list of audio files in the variable `paths2audio_files`.

**Collection**: ASR

# Changelog 
- Modified the `transcribe` method in the `EncDecCTCModel` class to support transcribing from manifest file. I first check that one of the arguments  `paths2audio_files` or `manifest_filepath` exists. If `manifest_filepath` is provided, add it to the `config` dict. Otherwise, create a temp manifest file from `paths2audio_files` list.

# Usage
* It can be combined with other models' output manifests, e.g. Lang ID, and use as an input to the ASR model using the `transcribe` method.


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- [X] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
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**PR Type**:
- [X] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
