Added python 3.12 dependency in CI

[avinashsharma080]

Fixes #3206

Description: Added python 3.12 to matrix in unit-test.yml and fixed extra whitespace in mnist.py

[vfdev-5]

@avinashsharma080 as you said on discord, py312 is failing on some nlp metrics due to removed _normalize arg in Fraction:

return Fraction(numerator, denominator, _normalize=False)

TypeError: Fraction.__new__() got an unexpected keyword argument '_normalize'

Can you remove _normalize arg everywhere it is used to see if it is really needed?

- return Fraction(numerator, denominator, _normalize=False)
+ return Fraction(numerator, denominator)

Actually, it is nltk's code and not ignite.

[vfdev-5]

Thanks @avinashsharma080