[Mellanox] Provide dummy implementation for get_rx_los and get_tx_fault (sonic-net#12231)

- Why I did it
get_rx_los and get_tx_fault is not supported via the exisitng interface used, need provide dummy implementation for them.
NOTE: in later releases we will get them back via different interface.

- How I did it
Return False * lane_num for get_rx_los and get_tx_fault

- How to verify it
Added unit test

Author: Junchao-Mellanox

platform/mellanox/mlnx-platform-api/sonic_platform/sfp.py

@@ -755,6 +755,38 @@ def get_error_description(self):
             error_description = "Unknow SFP module status ({})".format(oper_status)
         return error_description
 
+    def get_rx_los(self):
+        """Accessing rx los is not supproted, return all False
+
+        Returns:
+            list: [False] * channels
+        """
+        api = self.get_xcvr_api()
+        return [False] * api.NUM_CHANNELS if api else None
+
+    def get_tx_fault(self):
+        """Accessing tx fault is not supproted, return all False
+
+        Returns:
+            list: [False] * channels
+        """
+        api = self.get_xcvr_api()
+        return [False] * api.NUM_CHANNELS if api else None
+
+    def get_xcvr_api(self):
+        """
+        Retrieves the XcvrApi associated with this SFP
+
+        Returns:
+            An object derived from XcvrApi that corresponds to the SFP
+        """
+        if self._xcvr_api is None:
+            self.refresh_xcvr_api()
+            if  self._xcvr_api is not None:
+                self._xcvr_api.get_rx_los = self.get_rx_los
+                self._xcvr_api.get_tx_fault = self.get_tx_fault
+        return self._xcvr_api
+
 
 class RJ45Port(NvidiaSFPCommon):
     """class derived from SFP, representing RJ45 ports"""

platform/mellanox/mlnx-platform-api/tests/test_sfp.py

@@ -119,3 +119,17 @@ def test_is_port_admin_status_up(self, mock_port_status):
 
         mock_port_status.return_value = (0, False)
         assert not SFP.is_port_admin_status_up(None, None)
+
+    @mock.patch('sonic_platform.sfp.SFP.get_xcvr_api')
+    def test_dummy_apis(self, mock_get_xcvr_api):
+        mock_api = mock.MagicMock()
+        mock_api.NUM_CHANNELS = 4
+        mock_get_xcvr_api.return_value = mock_api
+
+        sfp = SFP(0)
+        assert sfp.get_rx_los() == [False] * 4
+        assert sfp.get_tx_fault() == [False] * 4
+
+        mock_get_xcvr_api.return_value = None
+        assert sfp.get_rx_los() is None
+        assert sfp.get_tx_fault() is None