Fix float/integer type error in WER.update()



# What does this PR do ?

Without this, `WER.update()` fails with the following exception:

    TypeError: 'float' object cannot be interpreted as an integer

This is because the `score` property is initialized as `tourch.int64`,
but actually is a float value. Fix the initialization so that the
property has the correct type.

*Technical Note:* the actual failing line is line 780 in `nemo/collections/asr/metrics/wer.py`:

```python3
self.scores = torch.tensor(scores, device=self.scores.device, dtype=self.scores.dtype)
```

Since `self.scores.dtype` is `torch.int64` in the current version, this line ended
up something like `torch.tensor(0.5, dtype=torch.int64)`  and fails.

**Collection**: [Note which collection this PR will affect]

ASR

# Changelog 

- Fix TypeError in WER.update()

# Usage

N/A

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
