Assert RAFT input resolution is 128 x 128 or higher (#7339)

Co-authored-by: Nicolas Hug <nicolashug@meta.com>

torchvision/models/optical_flow/raft.py

@@ -369,6 +369,19 @@ class CorrBlock(nn.Module):
             raise ValueError(
                 f"Input feature maps should have the same shape, instead got {fmap1.shape} (fmap1.shape) != {fmap2.shape} (fmap2.shape)"
             )
+
+        # Explaining min_fmap_size below: the fmaps are down-sampled (num_levels - 1) times by a factor of 2.
+        # The last corr_volume most have at least 2 values (hence the 2* factor), otherwise grid_sample() would
+        # produce nans in its output.
+        min_fmap_size = 2 * (2 ** (self.num_levels - 1))
+        if any(fmap_size < min_fmap_size for fmap_size in fmap1.shape[-2:]):
+            raise ValueError(
+                "Feature maps are too small to be down-sampled by the correlation pyramid. "
+                f"H and W of feature maps should be at least {min_fmap_size}; got: {fmap1.shape[-2:]}. "
+                "Remember that input images to the model are downsampled by 8, so that means their "
+                f"dimensions should be at least 8 * {min_fmap_size} = {8 * min_fmap_size}."
+            )
+
         corr_volume = self._compute_corr_volume(fmap1, fmap2)
 
         batch_size, h, w, num_channels, _, _ = corr_volume.shape  # _, _ = h, w