dev(BaseMetric): add docstring in self.add

mmeval/core/base_metric.py

@@ -126,8 +126,8 @@ def compute(self, size: Optional[int] = None) -> Dict:
 
  # NOTE: Needs discussion or investigation @yancong at 9/13/2022, 4:20:48 PM # noqa: E501
  # If the intermediate results stored in ``self._results`` are not
- # correspond one-to-one with samples (e.g. confusion matrix), the size
- # here may not work anymore.
+ # correspond one-to-one with samples (e.g. a total confusion matrix),
+ # the size here may not work anymore.
  if size is not None:
  collected_results = collected_results[:size]
 
@@ -145,8 +145,12 @@ def add(self, *args, **kwargs):
  """Override this method to add the intermediate results to
  ``self._results``.
 
- For performance issue, what you add to the ``self._results`` should be
- as simple as possible.
+ Note:
+ For performance issues, what you add to the ``self._results``
+ should be as simple as possible. But be aware that the intermediate
+ result stored in ``self._results`` should correspond one-to-one
+ with the samples, in that we need to remove the padded samples for
+ the most accurate result.
  """
 
  @abstractmethod