Token count and sequence length logging for MegatronGPTSFTModel
@vysarge 
Are you going to open this PR? We need this update, right? 
Let's merge this PR ASAP as the change looks to be straightforward. 

@vysarge 
Please change the merge target to the main branch.
# What does this PR do ?

Adds progress bar logging for sequence length (including padding tokens) and real token count for each train step with MegatronGPTSFTModel to enable accurate calculation of tokens/sec and related metrics.  Depends upon #7945.

**Collection**: nlp/language_modeling

# Changelog 
- Alters GPTSFTDataset and GPTSFTPackedDataset to pass out a list of token counts as part of the batch
- Alters MegatronGPTSFTModel to expect, average, and log that list of token counts at the beginning of the step

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

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**PR Type**:
- [x] New Feature
- [ ] Bugfix
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# Additional Information
* Depends upon #7945.
