Adding default identity implementations to InferenceSpec (#5278)

Co-authored-by: pintaoz-aws <167920275+pintaoz-aws@users.noreply.github.com>

Author: aviruthen

src/sagemaker/serve/spec/inference_spec.py

@@ -30,9 +30,11 @@ def invoke(self, input_object: object, model: object):
 
     def preprocess(self, input_data: object):
         """Custom pre-processing function"""
+        return input_data
 
     def postprocess(self, predictions: object):
         """Custom post-processing function"""
+        return predictions
 
     def prepare(self, *args, **kwargs):
         """Custom prepare function"""