Expose keep_initializers_as_inputs to Exportable class
# What does this PR do ?

[`torch.onnx.export`](https://pytorch.org/docs/stable/onnx.html#torch.onnx.export) has the parameter `keep_initializers_as_inputs` which allows to conveniently override parameters when running a graph.

**Collection**: Every collection that is exportable.

# Changelog 
- Allow to use `keep_initializers_as_inputs` parameter when exporting a model to ONNX

# Usage
* You can potentially add a usage example below

```python
model.export(..., keep_initializers_as_inputs=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


## Who can review?

@ericharper, @titu1994, @blisc, or @okuchaiev

# Additional Information