Update dynamic env handling to preserve None when USE_DYNAMIC is unset (#3567)

* Update dynamic env handling to preserve None when USE_DYNAMIC is unset

* Apply suggestions from code review

---------

Co-authored-by: Ilyas Moutawwakil <[email protected]>

Author: yafshar

src/accelerate/utils/dataclasses.py

@@ -1020,7 +1020,7 @@ def __post_init__(self):
             self.mode = os.environ.get(prefix + "MODE", "default")
         if self.fullgraph is None:
             self.fullgraph = str_to_bool(os.environ.get(prefix + "USE_FULLGRAPH", "False")) == 1
-        if self.dynamic is None:
+        if self.dynamic is None and os.environ.get(prefix + "USE_DYNAMIC", None) is not None:
             self.dynamic = str_to_bool(os.environ.get(prefix + "USE_DYNAMIC", "False")) == 1
 
     def to_dict(self):