Fix do_eval default value in training_args.py
# What does this PR do?

According to <do_eval> description, when <evaluation_strategy> is different from 'no' it will be set 'True'.
But the <do_eval>'s defualt setting is None. So, this code can't be executed unless the the user set <do_eval = False>.

`if self.do_eval is False and self.evaluation_strategy != IntervalStrategy.NO : self.do_eval = True  `

I think it will be better to change <do_eval>'s defulat value 'None' into 'False'

- How I FOUND IT.
I was trying to use <training_args.do_eval> in my script.
BUT it didn't worked even the <evaluation_strategy> was set to 'steps'.
