Fix, remove last comma

ipu_config.json

@@ -10,5 +10,5 @@
     "replicated_tensor_sharding": true,
     "device_iterations": 10,                                                                                                      
     "gradient_accumulation_steps": 128,
-    "executable_cache_dir": "./exe_cache",                                                                                       
+    "executable_cache_dir": "./exe_cache"                                                                                       
 }