Fix distributions/test_distributions.py for Python 3.10
This is a partial fix for https://github.com/pytorch/pytorch/issues/66424
Without this PR, on Python 3.10 `python run_test.py -i distributions/test_distributions` fails with "TypeError: 'float' object cannot be interpreted as an integer".
The change is only needed for StudenT distribution because it has default float parameters.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74007
Approved by: https://github.com/ezyang
