Replace PyGrainEncDecFeatureConverter with T5XEncDecFeatureConverter.

PiperOrigin-RevId: 598889947

t5x/utils.py

@@ -32,7 +32,7 @@
 from absl import flags
 from absl import logging
 import airio
-from airio.grain import feature_converters
+from airio.grain.common import feature_converters
 import clu.data
 import flax
 from flax import traverse_util