add rampup batch size support for Megatron GPT
# What does this PR do ?

This PR adds ramp up batch size support for GPT models. It allows to increase the batch size up to global batch size linearly during training in a given number of samples.

**Collection**: NLP

# Changelog 
- Add rampup_batch_size value to the training config.
- Input rampup_batch_size to the apex microbatch_calculator.
- Add current global_batch_size value to the progress bar when using rampup_batch_size.
- Update the apex microbatch_calculator after each training step.

# Usage
Can be set via config or CLI.

**Config:**
```python
rampup_batch_size: [8, 4, 10000] # Should be a list of 3 values: [<start_batch_size>, <batch_size_increment>, <rampup_samples>]
```
**CLI:**
```python
model.rampup_batch_size=[8,4,10000]
```
# 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)
- [x] 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)
