Add an option to defer data setup from ``__init__`` to ``setup``
Signed-off-by: Ante Jukić <ajukic@nvidia.com>

# What does this PR do ?

This PR adds an option to defer dataloader setup in `ModelPT` from `__init__` to `setup`.
Default behavior remains unchanged.

**Collection**: Core

# Changelog 
- In `ModelPT::__init__()`
  - Nothing is changed if `self._cfg.train_ds.defer_setup` is not set or is set to `False` (default)
  - If `self._cfg.train_ds.defer_setup` is set to `True`, `self.setup_training_data` will be skipped
  - Analogous behavior for validation and test data

- In `ModelPT::setup()`
  - If dataloader is not initialized and `defer_setup==True`, run data setup

# Usage

To use deferred setup, set the following

```python
  cfg.model.train_ds.defer_setup = True
  cfg.model.validation_ds.defer_setup = True
  cfg.model.test_ds.defer_setup = True
```

# 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**:
- [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)
