rm unnecessary out_id variable
Hi @rohan100jain Can you please review this PR ? Thank you! Hi @rohan100jain Can you please review this PR ? Thank you! Thanks for the PR. However I don't think this is necessary for a bugfix or improves readability of the code. 
Don't think need to assign variable `int out_id = out->id();`, if only for consistency with rest of `tensorflow/core/graph/control_flow.cc`