Support of unnamed saved_model_dir in MO Python API (#16542)

* Added support of unnamed saved_model_dir.

* Switch TF2 layer tests for unnamed saved_model_dir.

* Added test.

* Correction of comment.

* Removed unnecessary pytest mark.

* Code correction, added comment.

tools/mo/openvino/tools/mo/convert_impl.py

@@ -808,6 +808,22 @@ def pack_params_to_args_namespace(args: dict, cli_parser: argparse.ArgumentParse
     return argv
 
 
+def update_args_for_saved_model_dir(args: dict):
+    """
+    If directory is set in 'input_model' argument, the directory is considered as TF saved model.
+    In this case this method updates args and moves saved model directory to 'saved_model_dir' param.
+    :param args: dictionary with arguments from user
+    """
+    if 'saved_model_dir' in args and args['saved_model_dir'] is not None and \
+            'input_model' in args and args['input_model'] is not None:
+        raise Error("Both --input_model and --saved_model_dir are defined. "
+                    "Please specify either input_model or saved_model_dir directory.")
+    
+    if 'input_model' in args and isinstance(args['input_model'], (str, Path)) and os.path.isdir(args['input_model']):
+        args['saved_model_dir'] = args['input_model']
+        args['input_model'] = None
+
+
 def _convert(cli_parser: argparse.ArgumentParser, framework, args):
     if 'help' in args and args['help']:
         show_mo_convert_help()
@@ -839,6 +855,8 @@ def _convert(cli_parser: argparse.ArgumentParser, framework, args):
                 args['input_model'] = decoder
                 args["framework"] = "pytorch"
 
+        update_args_for_saved_model_dir(args)
+
         argv = pack_params_to_args_namespace(args, cli_parser)
 
         argv.feManager = FrontEndManager()