TTS Aligner Inference Example outputs attn_soft_tensor with nan values 
**Describe the bug**

I simply followed the [TTS Aligner Inference Example](https://github.com/NVIDIA/NeMo/blob/stable/tutorials/tts/Aligner_Inference_Examples.ipynb) but the visualized `attn_soft` is empty and indeed `attn_soft_tensor` has all `nan` values.

<img width="1082" alt="Screen Shot 2023-05-06 at 10 43 29 PM" src="https://user-images.githubusercontent.com/621009/236655018-9c65c525-684c-42f6-ba1a-45e67024113f.png">

**Steps/Code to reproduce bug**

You can simply proceed through [this notebook]((https://github.com/NVIDIA/NeMo/blob/stable/tutorials/tts/Aligner_Inference_Examples.ipynb)

**Expected behavior**

Display a graph with values described in the instruction.

**Environment overview (please complete the following information)**

Google Collab