[fx/graph_manipulation] Fix _update_weight_fused_dtypes (#77702)
Summary: D36335238 (https://github.com/pytorch/pytorch/commit/18e36a6295ebc4ff4de69e14c71a15619eb69bcb) wasn't fully working due to previous impl which used op names for looking for matches. Instead use the FX graph itself.
Differential Revision: D36462875
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77702
Approved by: https://github.com/jamesr66a
