Concat samples
# What does this PR do ?

Adds [limited] support for runtime concatenation of samples

**Collection**: ASR

# Changelog 
- The implementation is limited to the ConcatDataset class. 
- There is no sample normalization
- samples are joined with a zero pause

# Usage
* You can potentially add a usage example below

```python
# Add these parameters
train_ds.is_concat: True  # need to enable the concat dataset
train_ds.concat_samples: True  # enable sample concatenation
train_ds.concat_samples_count_as_one: True  # count samples as per real number or  as one
train_ds.concat_samples_max_length: 22 # max length of concatenated sample
train_ds.concat_samples_min_length: 16 # min length of concatenated sample
train_ds.concat_samples_joining_pause: 0.1 # joining pause
```

# 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**:
- [X ] New Feature
- [ ] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## 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)
