Ensure that _hashable_filter does not convert strings to a tuple of letters

PiperOrigin-RevId: 588355403

Author: superbobry

flax/core/lift.py

@@ -1491,6 +1491,8 @@ def rematted(variable_groups, rng_groups, *args, **kwargs):
 
 def _hashable_filter(x):
   """Hashable version of CollectionFilter."""
+  if isinstance(x, str):
+    return (x,)
   if isinstance(x, Iterable):
     return tuple(x)  # convert un-hashable list & sets to tuple
   if isinstance(x, DenyList):