perf: skip bincount if unnecessary (#4773)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Improved the aggregation logic to conditionally compute bin counts,
enhancing accuracy when averaging or specifying the number of owners.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: caic99

deepmd/pt/model/network/utils.py

@@ -30,17 +30,22 @@ def aggregate(
     -------
     output: [num_owner, feature_dim]
     """
-    bin_count = torch.bincount(owners)
-    bin_count = bin_count.where(bin_count != 0, bin_count.new_ones(1))
-
-    if (num_owner is not None) and (bin_count.shape[0] != num_owner):
-        difference = num_owner - bin_count.shape[0]
-        bin_count = torch.cat([bin_count, bin_count.new_ones(difference)])
-
-    # make sure this operation is done on the same device of data and owners
-    output = data.new_zeros([bin_count.shape[0], data.shape[1]])
+    if num_owner is None or average:
+        # requires bincount
+        bin_count = torch.bincount(owners)
+        bin_count = bin_count.where(bin_count != 0, bin_count.new_ones(1))
+        if (num_owner is not None) and (bin_count.shape[0] != num_owner):
+            difference = num_owner - bin_count.shape[0]
+            bin_count = torch.cat([bin_count, bin_count.new_ones(difference)])
+        else:
+            num_owner = bin_count.shape[0]
+    else:
+        bin_count = None
+
+    output = data.new_zeros([num_owner, data.shape[1]])
     output = output.index_add_(0, owners, data)
     if average:
+        assert bin_count is not None
         output = (output.T / bin_count).T
     return output