Changes to support mean n-gram size masking for T5
Signed-off-by: MaximumEntropy <sandeep.subramanian.1@umontreal.ca>

# What does this PR do ?

Allows configuring the mean n-gram size for T5 span masking when using a geometric distribution.

**Collection**: NLP

# Changelog 
- Used the fact that the expectation of the geometric distribution is 1/p to set p appropriately for a given expected value of n-gram size.

# Usage

To get a mean n-gram size of 3 while masking (like suggested in the T5 paper), thee data configuration for T5 can look like this:

```
    max_ngram_size: ${seq_length_dec}
    mean_ngram_size: 3
    geometric_dist: True
    permutation: False
    whole_work_masking: True
    favor_longer_ngrams: False
```

```python
# N/A same run script as T5, just requires changing the yaml config like above.
```

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**PR Type**:
- [ ] New Feature
- [ ] Bugfix
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# Additional Information
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