Refa: PARALLEL_DEVICES is a static parameter. (#6168)

### What problem does this PR solve?


### Type of change

- [x] Refactoring

rag/settings.py

@@ -39,6 +39,13 @@ SVR_CONSUMER_GROUP_NAME = "rag_flow_svr_task_broker"
 PAGERANK_FLD = "pagerank_fea"
 TAG_FLD = "tag_feas"
 
+PARALLEL_DEVICES = None
+try:
+    import torch.cuda
+    PARALLEL_DEVICES = torch.cuda.device_count()
+    logging.info(f"found {PARALLEL_DEVICES} gpus")
+except Exception:
+    logging.info("can't import package 'torch'")
 
 def print_rag_settings():
     logging.info(f"MAX_CONTENT_LENGTH: {DOC_MAXIMUM_SIZE}")