[dynamo] Optimize VariableTracker.__post_init__ (#122034)
Improves `benchmarks/dynamo/microbenchmarks/dynamo_microbenchmarks.py`
from 8.6s to 7.3s.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122034
Approved by: https://github.com/Skylion007
ghstack dependencies: #122032, #122033
