Fix initializing weights from ptl ckpt with exclude
Signed-off-by: sam1373 <samuelkriman@gmail.com>

# What does this PR do ?

# Changelog 
- fix typo "model_path" to "ckpt_path"
- also makes the "load_from_string" parameter optional

**PR Type**:
- [ ] New Feature
- [X] Bugfix
- [ ] Documentation


# Additional Information
* Related to [#4805](https://github.com/NVIDIA/NeMo/issues/4805)
