correct NLLLoss parameters default value (#68426)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/17577
Previous
`size_average by default: True`
`reduce by default: True`
Present
`size_average by default: None`
`reduce by default: None`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68426
Reviewed By: VitalyFedyunin
Differential Revision: D32463324
Pulled By: jbschlosser
fbshipit-source-id: 7ba9cd03c9fb6b2f19301e7e39c3c490de17202b
