Fix seeding in neural machine translation examples
Signed-off-by: sarthmit <sarthmit@gmail.com>

# What does this PR do ?

Fixes the seed for everything (training/initialization/etc.) during training of nlp machine translation systems to aid reproducibility.

**Collection**: examples/nlp/machine_translation

# Changelog 
- Using pytorch_lightning's seed_everything function to fix the seed within each training file for machine translation in nlp.
- Update the config files to start with default seed as 0

# Usage
* Usage is as per earlier, where the seed can be changed by just adding a seed=[seed] to override the default 0 in the configs.

**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests? 
- [x] Did you add or update any necessary documentation?
- [x] 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

## Who can review? 
@MaximumEntropy @okuchaiev