[Feature] Add deterministic_sample to masked categorical

ghstack-source-id: a6506ac99a83816ee3a1dbf44ce2795889310cdd
Pull Request resolved: #2708

Author: vmoens

torchrl/modules/distributions/discrete.py

@@ -319,6 +319,10 @@ def _mask_logits(
             logits.masked_fill_(padding_mask, neg_inf)
         return logits
 
+    @property
+    def deterministic_sample(self):
+        return self.mode
+
 
 class MaskedOneHotCategorical(MaskedCategorical):
     """MaskedCategorical distribution.