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

@@ -61,7 +61,7 @@ class RAGFlowPdfParser:
 
         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"):