String comparison with "is" 

See https://github.com/deepset-ai/FARM/blob/0357634c1997fe39b6f6e78827d35e43538caab1/farm/modeling/adaptive_model.py#L583

Should replace "is" with "=="

I do not think this is likely to cause a problem, but it depends on the way how Python string interning works. 