[Enhancement] Change assertion logic inference cfg.model.test_cfg (open-mmlab#3012)

## Motivation

In encode_decoder.py , assertion logic is not working correctly if user
modifes cfg.test_cfg and defines it in a dictionary format. See:
open-mmlab#3011

## Modification

Slight change to assertion behaviour to change assertion depending on if
received test_cfg object is a dict or not.

## BC-breaking (Optional)

Unsure - I believe this will not break any downstream tasks as the
previous logic is still included

## Use cases (Optional)

n/a

---------

Co-authored-by: xiexinch <[email protected]>

Author: LHamnett

mmseg/models/segmentors/encoder_decoder.py

@@ -33,7 +33,7 @@ class EncoderDecoder(BaseSegmentor):
     2. The ``predict`` method is used to predict segmentation results,
     which includes two steps: (1) Run inference function to obtain the list of
     seg_logits (2) Call post-processing function to obtain list of
-    ``SegDataSampel`` including ``pred_sem_seg`` and ``seg_logits``.
+    ``SegDataSample`` including ``pred_sem_seg`` and ``seg_logits``.
 
     .. code:: text
 
@@ -326,8 +326,9 @@ def inference(self, inputs: Tensor, batch_img_metas: List[dict]) -> Tensor:
             Tensor: The segmentation results, seg_logits from model of each
                 input image.
         """
-
-        assert self.test_cfg.mode in ['slide', 'whole']
+        assert self.test_cfg.get('mode', 'whole') in ['slide', 'whole'], \
+            f'Only "slide" or "whole" test mode are supported, but got ' \
+            f'{self.test_cfg.mode}'
         ori_shape = batch_img_metas[0]['ori_shape']
         assert all(_['ori_shape'] == ori_shape for _ in batch_img_metas)
         if self.test_cfg.mode == 'slide':