I got 1083 rank in CPGET. I was not allotted M.Sc nutrition and dietician in 1st phase of counselling. Should I go for 2nd phase?

You can fill the fresh choices again and go for 2nd round of counseling, as there are high chances of getting a seat as almost 40% of candidates don't join and would be opting for a government college in their state round.

Those who are not allotted seat in the first round but are already registered

Yes,

you should definitely go for the second phase of CPGET counselling if you are interested in M.Sc. Nutrition and Dietetics.

Your rank of 1083 is quite good, and you have a good chance of being allotted a seat in the second phase.