[tune] Function API checkpointing (#8471)

Co-authored-by: krfricke <krfricke@users.noreply.github.com>

python/ray/tune/experiment.py

@@ -3,6 +3,7 @@ import logging
 import os
 
 from ray.tune.error import TuneError
+from ray.tune.function_runner import detect_checkpoint_function
 from ray.tune.registry import register_trainable, get_trainable_cls
 from ray.tune.result import DEFAULT_RESULTS_DIR
 from ray.tune.sample import sample_from
@@ -92,6 +93,18 @@ class Experiment:
                  restore=None):
 
         config = config or {}
+
+        if callable(run) and detect_checkpoint_function(run):
+            if checkpoint_at_end:
+                raise ValueError(
+                    "'checkpoint_at_end' cannot be used with a "
+                    "checkpointable function. You can specify and register "
+                    "checkpoints within your trainable function.")
+            if checkpoint_freq:
+                raise ValueError(
+                    "'checkpoint_freq' cannot be used with a "
+                    "checkpointable function. You can specify checkpoints "
+                    "within your trainable function.")
         self._run_identifier = Experiment.register_if_needed(run)
         self.name = name or self._run_identifier
         if upload_dir: