[Doc]: Update description of Args: voxel_features (#1614)

Motivation
Just better description in PointPillarScatter.

Modification
The original description of voxel_features is (N, M, C), which indeed is (N, C)

BC-breaking (Optional)
NO

Use cases (Optional)
NO

Checklist
Pre-commit or other linting tools are used to fix the potential lint issues.
The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
The documentation has been modified accordingly, like docstring or example tutorials.

mmdet3d/models/middle_encoders/pillar_scatter.py

@@ -39,7 +39,7 @@ def forward_single(self, voxel_features, coors):
         """Scatter features of single sample.
 
         Args:
-            voxel_features (torch.Tensor): Voxel features in shape (N, M, C).
+            voxel_features (torch.Tensor): Voxel features in shape (N, C).
             coors (torch.Tensor): Coordinates of each voxel.
                 The first column indicates the sample ID.
         """
@@ -63,7 +63,7 @@ def forward_batch(self, voxel_features, coors, batch_size):
         """Scatter features of single sample.
 
         Args:
-            voxel_features (torch.Tensor): Voxel features in shape (N, M, C).
+            voxel_features (torch.Tensor): Voxel features in shape (N, C).
             coors (torch.Tensor): Coordinates of each voxel in shape (N, 4).
                 The first column indicates the sample ID.
             batch_size (int): Number of samples in the current batch.