set PARALLEL_DEVICES default value= 0 (#7935)

### What problem does this PR solve?


it would be fail if PARALLEL_DEVICES = None in OCR class , because it
pass 0 to TextDetector and TextRecognizer init method.

and It would be simpler to set 0 as the default value for
PARALLEL_DEVICES.

### Type of change

- [x] Refactoring

deepdoc/parser/pdf_parser.py

 
         self.ocr = OCR()
         self.parallel_limiter = None
-        if PARALLEL_DEVICES is not None and PARALLEL_DEVICES > 1:
+        if PARALLEL_DEVICES > 1:
             self.parallel_limiter = [trio.CapacityLimiter(1) for _ in range(PARALLEL_DEVICES)]
 
         if hasattr(self, "model_speciess"):

deepdoc/vision/ocr.py

                         "rag/res/deepdoc")
                 
                 # Append muti-gpus task to the list
-                if PARALLEL_DEVICES is not None and PARALLEL_DEVICES > 0:
+                if PARALLEL_DEVICES > 0:
                     self.text_detector = []
                     self.text_recognizer = []
                     for device_id in range(PARALLEL_DEVICES):
                         self.text_detector.append(TextDetector(model_dir, device_id))
                         self.text_recognizer.append(TextRecognizer(model_dir, device_id))
                 else:
-                    self.text_detector = [TextDetector(model_dir, 0)]
-                    self.text_recognizer = [TextRecognizer(model_dir, 0)]
+                    self.text_detector = [TextDetector(model_dir)]
+                    self.text_recognizer = [TextRecognizer(model_dir)]
 
             except Exception:
                 model_dir = snapshot_download(repo_id="InfiniFlow/deepdoc",
                                               local_dir=os.path.join(get_project_base_directory(), "rag/res/deepdoc"),
                                               local_dir_use_symlinks=False)
                 
-                if PARALLEL_DEVICES is not None:
-                    assert PARALLEL_DEVICES > 0, "Number of devices must be >= 1"
+                if PARALLEL_DEVICES > 0:
                     self.text_detector = []
                     self.text_recognizer = []
                     for device_id in range(PARALLEL_DEVICES):
                         self.text_detector.append(TextDetector(model_dir, device_id))
                         self.text_recognizer.append(TextRecognizer(model_dir, device_id))
                 else:
-                    self.text_detector = [TextDetector(model_dir, 0)]
-                    self.text_recognizer = [TextRecognizer(model_dir, 0)]
+                    self.text_detector = [TextDetector(model_dir)]
+                    self.text_recognizer = [TextRecognizer(model_dir)]
 
         self.drop_score = 0.5
         self.crop_image_res_index = 0

rag/settings.py

 PAGERANK_FLD = "pagerank_fea"
 TAG_FLD = "tag_feas"
 
-PARALLEL_DEVICES = None
+PARALLEL_DEVICES = 0
 try:
     import torch.cuda
     PARALLEL_DEVICES = torch.cuda.device_count()