PipelineMixedPrecisionPlugin: dtype is `None`
In [this logic](https://github.com/NVIDIA/NeMo/blob/c56119623a3ad1a0b2e2767f88b65ddcac0cbd1b/nemo/collections/nlp/parts/nlp_overrides.py#L549), dtype might be `None` and causes errors like:

```
Traceback (most recent call last):
  File "/tmp/nvflare/nemo/peft_fedavg_345M/simulate_job/app_site-1/custom/megatron_gpt_peft_tuning.py", line 203, in main
    plugins.append(PipelineMixedPrecisionPlugin(precision=plugin_precision, device='cuda', scaler=scaler))
  File "/usr/local/lib/python3.10/dist-packages/nemo/collections/nlp/parts/nlp_overrides.py", line 525, in __init__
    torch.set_autocast_gpu_dtype(dtype)
TypeError: dtype must be a torch.dtype (got NoneType)
```

There should be a informative error message in case dtype is not set correctly.