Fix Megatron Bert Broadcast when using Binary Head
# What does this PR do ?

Fixes gpu hang caused by incompatible shapes during loss mean broadcast when binary head is on.  This may not be the cleanest solution but I am making a draft PR for awareness of the issue.

**Collection**: NLP

# Changelog 
- This adds an if statement to broadcast an shape [3] zeros tensor for loss mean reduction when the Binary Head is on.

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- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
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**PR Type**:
- [Bugfix]


# Additional Information
* cc. @shanmugamr1992 
