Add safe_serialization (safetensors) option to the checkpoint conversion script (`examples/convert_dcp_to_hf.py`)

**Is your feature request related to a problem? Please describe.**

Currently, the ckpt conversion script `examples/convert_dcp_to_hf.py` saves model weights in `.bin` for the HF checkpoint. The user has to convert it to safetensors.

**Describe the solution you'd like**

HF has a feature. It should be something like add an option

```
model.save_pretrained(
...,
safe_serialization=True
)
```

https://github.com/NVIDIA/NeMo-RL/blob/main/examples/convert_dcp_to_hf.py

**Describe alternatives you've considered**

**Additional context**