Allow relative paths in manifest
# What does this PR do ?

 Allow to use relative paths in manifest file.

**Collection**: ASR or any other collection which use manifest file with field "audio_file"  

# Changelog 
- Allow to use relative paths in the ASR manifest file 

# Usage
* Allow to use relative paths in the manifest file. Field audio_file may be relative to the folder of manifest file


# Before your PR is "Ready for review"
**Pre checks**:
- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
