Add Metric.from_mask helper method (facebookresearch#3411)

Author: poojasethi

parlai/core/metrics.py

@@ -24,6 +24,7 @@
     Optional,
     Set,
     Tuple,
+    Type,
     Union,
 )
 
@@ -272,7 +273,7 @@ def many(cls, *objs: List[TVector]) -> List[Metric]:
         """
         Construct many of a Metric from the base parts.
 
-        Useful if you separately compute numerators and denomenators, etc.
+        Useful if you separately compute numerators and denominators, etc.
         """
         lengths = [len(o) for o in objs]
         objs = list(objs)  # convert from tuple for inplace modification
@@ -286,6 +287,27 @@ def many(cls, *objs: List[TVector]) -> List[Metric]:
             raise IndexError(f'Uneven {cls.__name__} constructions: {lengths}')
         return [cls(*items) for items in zip(*objs)]
 
+    @classmethod
+    def from_mask(
+        cls, metric_per_token: torch.Tensor, mask: torch.Tensor, MyMetric: Type[Metric]
+    ) -> List[Metric]:
+        """
+        From token-level metrics, returns aggregate MyMetric per example in the batch.
+
+        :param metric_per_token:
+            a (batchsize x num_tokens) Tensor
+        :param mask:
+            a (batchsize x num_tokens) Tensor to mask out tokens that should *not* be considered in the aggregate metric calculation.
+        :param MyMetric:
+            a subclass of Metric
+        :return:
+            a (batchsize) Tensor
+        """
+        tokens_per_ex = mask.long().sum(dim=-1)
+        metric_per_ex = (metric_per_token * mask).sum(dim=-1)
+        metrics = MyMetric.many(metric_per_ex, tokens_per_ex)
+        return metrics
+
 
 class FixedMetric(Metric):
     """

parlai/core/torch_generator_agent.py

@@ -34,7 +34,7 @@
 from parlai.utils.misc import warn_once
 from parlai.utils.io import PathManager
 import parlai.utils.logging as logging
-from parlai.core.metrics import SumMetric, AverageMetric, FairseqBleuMetric
+from parlai.core.metrics import Metric, SumMetric, AverageMetric, FairseqBleuMetric
 from parlai.utils.fp16 import FP16SafeCrossEntropy
 import parlai.utils.fsdp as fsdp_utils
 from parlai.utils.torch import (
@@ -710,28 +710,35 @@ def compute_loss(self, batch, return_output=False):
         model_output = self.model(*self._model_input(batch), ys=batch.label_vec)
         scores, preds, *_ = model_output
         score_view = scores.reshape(-1, scores.size(-1))
-        loss = self.criterion(score_view, batch.label_vec.view(-1))
-        loss = loss.view(scores.shape[:-1]).sum(dim=1)
-        # save loss to metrics
+        loss_flattened = self.criterion(score_view, batch.label_vec.view(-1))
+        loss_per_token = loss_flattened.view(scores.shape[:-1])
         notnull = batch.label_vec.ne(self.NULL_IDX)
-        target_tokens = notnull.long().sum(dim=-1)
-        correct = ((batch.label_vec == preds) * notnull).sum(dim=-1)
 
+        # save loss to metrics
         # cross entropy loss
-        self.record_local_metric('loss', AverageMetric.many(loss, target_tokens))
+        self.record_local_metric(
+            'loss', Metric.from_mask(loss_per_token, notnull, AverageMetric)
+        )
         # perplexity
-        self.record_local_metric('ppl', PPLMetric.many(loss, target_tokens))
+        self.record_local_metric(
+            'ppl', Metric.from_mask(loss_per_token, notnull, PPLMetric)
+        )
         # token-wise accuracy
         self.record_local_metric(
-            'token_acc', AverageMetric.many(correct, target_tokens)
+            'token_acc',
+            Metric.from_mask(batch.label_vec == preds, notnull, AverageMetric),
         )
         # utterance-wise exact match
+        num_target_tokens = notnull.long().sum(dim=-1)
+        num_tokens_correct = ((batch.label_vec == preds) * notnull).sum(dim=-1)
         self.record_local_metric(
-            'token_em', AverageMetric.many(correct == target_tokens)
+            'token_em', AverageMetric.many(num_tokens_correct == num_target_tokens)
         )
+
         # actually do backwards loss
+        loss = loss_per_token.sum(dim=1)
         loss = loss.sum()
-        loss /= target_tokens.sum()  # average loss per token
+        loss /= num_target_tokens.sum()  # average loss per token
         if return_output:
             return (loss, model_output)
         else:
@@ -1440,7 +1447,7 @@ def set_block_list(self: TSType, block_list: Optional[SearchBlocklist]) -> TSTyp
 
     def get_output_from_current_step(self):
         """
-        Get the outputput at the current step.
+        Get the output at the current step.
         """
         return self.outputs[-1]