Add note about update

Author: rohan-varma

pytorch_lightning/overrides/data_parallel.py

@@ -164,6 +164,7 @@ def parallel_apply(self, replicas, inputs, kwargs):
         return parallel_apply(replicas, inputs, kwargs, self.device_ids[:len(replicas)])
 
     def forward(self, *inputs, **kwargs):  # pragma: no-cover
+        # TODO: Update uneven inputs code path when PyTorch 1.8 is released.
         if DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE and self.ddp_join_enabled:
             ones = torch.ones(
                 1, device=self.device