Binarized memmap dataloader for Megatron NMT, Inference and checkpoint -> nemo
# What does this PR do ?

1. Adds megatron memory-mapped dataloaders to NMT.
2. Inference script/config with a translate() method.

**Collection**: NLP

# Changelog 
- Add a new dataset class for megatron memmap dataset.
- Add an inference script with the associated yaml config.
- Change the `use_tarred_dataset` arg to a generic `dataset_type` arg that can take `[text, tarred, bin_memmap, text_memmap]`

# Usage
* Set the following in the yaml config

- `dataset_type: bin_memmap`.

# 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
- [ ] 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)
