add available backends

Signed-off-by: Behrooz <3968947+drbeh@users.noreply.github.com>

Author: bhashemian

monai/data/wsi_reader.py

@@ -322,6 +322,8 @@ class WSIReader(BaseWSIReader):
 
     """
 
+    available_backends = ["cucim", "openslide", "tifffile"]
+
     def __init__(
         self,
         backend="cucim",