Refactor get_num_dpus function. (#17601)

Refactor current get_num_dpus function to consuming platform.json which is reliable way to retrieve the number of DPUs.

src/sonic-py-common/sonic_py_common/device_info.py

@@ -847,23 +847,37 @@ def is_frontend_port_present_in_host():
 
 
 def get_num_dpus():
-    # Todo: we should use platform api to get the dpu number
-    # instead of rely on the platform env config.
-    num_dpus = 0
-    platform_env_conf_file_path = get_platform_env_conf_file_path()
+    """
+    Retrieves the number of DPUs from platform.json file.
 
-    # platform_env.conf file not present for platform
-    if platform_env_conf_file_path is None:
-        return num_dpus
+    Args:
 
-    # Else open the file check for keyword - num_dpu -
-    with open(platform_env_conf_file_path) as platform_env_conf_file:
-        for line in platform_env_conf_file:
-            tokens = line.split('=')
-            if len(tokens) < 2:
-               continue
-            if tokens[0].lower() == 'num_dpu':
-                num_dpus = tokens[1].strip()
-                break
-    return int(num_dpus)
+    Returns:
+        A integer to indicate the number of DPUs.
+    """
+
+    platform = get_platform()
+    if not platform:
+        return 0
+
+    # Get Platform path.
+    platform_path = get_path_to_platform_dir()
+
+    if os.path.isfile(os.path.join(platform_path, PLATFORM_JSON_FILE)):
+        json_file = os.path.join(platform_path, PLATFORM_JSON_FILE)
+        
+        try:
+            with open(json_file, 'r') as file:
+                platform_data = json.load(file)
+        except (json.JSONDecodeError, IOError, TypeError, ValueError):
+            # Handle any file reading and JSON parsing errors
+            return 0
+        
+        # Convert to lower case avoid case sensitive.
+        data = {k.lower(): v for k, v in platform_data.items()}
+        DPUs = data.get('dpus', None)
+        if DPUs is not None and len(DPUs) > 0:
+            return len(DPUs)
+
+    return 0