Fix FASF onnx export support (#4735)

mmdet/core/bbox/coder/tblr_bbox_coder.py

@@ -170,7 +170,9 @@ def tblr2bboxes(priors,
         w, h = torch.split(wh, 1, dim=-1)
         loc_decode[..., :2] *= h  # tb
         loc_decode[..., 2:] *= w  # lr
-    top, bottom, left, right = loc_decode.split(1, dim=-1)
+
+    # Cannot be exported using onnx when loc_decode.split(1, dim=-1)
+    top, bottom, left, right = loc_decode.split((1, 1, 1, 1), dim=-1)
     xmin = prior_centers[..., 0].unsqueeze(-1) - left
     xmax = prior_centers[..., 0].unsqueeze(-1) + right
     ymin = prior_centers[..., 1].unsqueeze(-1) - top