computeChains with nomnigraph (#15366)

computeChains with nomnigraph (#15366)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15366

swap the old implementation with a slightly easier one to understand

I ran the tests and compared the number of chains compared to the old algorithm.  This one outperforms on every test, but we have yet to see if that impacts performance at all.

old chain 34 nomnigraph chain 25
old chain 46 nomnigraph chain 34
old chain 228 nomnigraph chain 188
old chain 397 nomnigraph chain 338

Reviewed By: ilia-cher

Differential Revision: D13057451

fbshipit-source-id: ccd050bfead6eb94ab9c7b0a70b09a22c2b9e499