[rllib] Fix output API when lz4 not installed (#5421)

rllib/offline/json_writer.py

@@ -19,7 +19,7 @@ from ray.rllib.policy.sample_batch import MultiAgentBatch
 from ray.rllib.offline.io_context import IOContext
 from ray.rllib.offline.output_writer import OutputWriter
 from ray.rllib.utils.annotations import override, PublicAPI
-from ray.rllib.utils.compression import pack
+from ray.rllib.utils.compression import pack, compression_supported
 
 logger = logging.getLogger(__name__)
 
@@ -99,7 +99,7 @@ class JsonWriter(OutputWriter):
 
 
 def _to_jsonable(v, compress):
-    if compress:
+    if compress and compression_supported():
         return str(pack(v))
     elif isinstance(v, np.ndarray):
         return v.tolist()

rllib/utils/compression.py

@@ -23,6 +23,11 @@ except ImportError:
     LZ4_ENABLED = False
 
 
+@DeveloperAPI
+def compression_supported():
+    return LZ4_ENABLED
+
+
 @DeveloperAPI
 def pack(data):
     if LZ4_ENABLED: