Update loader

Signed-off-by: shiyu22 <shiyu.chen@zilliz.com>

__init__.py

@@ -0,0 +1,5 @@
+from .loader import TextLoader
+
+
+def text_loader(*args, **kwargs):
+    return TextLoader(*args, **kwargs)

loader.py

@@ -0,0 +1,16 @@
+from towhee.operator import PyOperator
+
+
+class TextLoader(PyOperator):
+    """
+    DefaultOperator for _input and _output nodes.
+    """
+
+    def __init__(self):
+        #pylint: disable=useless-super-delegation
+        super().__init__()
+
+    def __call__(self, *args):
+        if len(args) == 1:
+            return args[0]
+        return args