[Tutorial][ASR][Fix] Data paths in ASR with NeMo tutorial

# What does this PR do ?

This PR aims to fix a few small issues in the ASR with NeMo tutorial.

**Collection**: ASR

# Changelog 
- check if `data_dir` exists and create if necessary
- prepend `data_dir` to `paths2audio_files`
- check if the expected config file exists and download if necessary

# Usage
n/a

# Before your PR is "Ready for review"
**Pre checks**:
- [x] 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
- [x] Bugfix
- [ ] Documentation

## 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
n/a
