[dynamo] Optimize SourcelessBuilder (#122063)
Improves `benchmarks/dynamo/microbenchmarks/dynamo_microbenchmarks.py`
from 2.7s to 2.5s.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122063
Approved by: https://github.com/anijain2305
ghstack dependencies: #122039, #122043, #122055, #122058, #122060
