[2/N] More ruff SIM fixes (#165031)

This is follow-up of #164695 to apply ruff SIM rules to more files. Most changes are about simplifying dict.get because None is already the default value.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/165031
Approved by: https://github.com/mlazos

torch/optim/optimizer.py

@@ -783,8 +783,8 @@ class Optimizer:
         assert param_groups is not None
         for pg in param_groups:
             if param_id in pg["params"]:
-                fused = pg["fused"] if "fused" in pg else False
-                capturable = pg["capturable"] if "capturable" in pg else False
+                fused = pg.get("fused", False)
+                capturable = pg.get("capturable", False)
                 break
         if key == "step":
             if capturable or fused: