Small bug in FreeAnchor head (#944)

* reduce 1e-12 to make difference between variables

If 1e-12 is used to make a difference between t1 and t2, then later (t2-t1) might become 0 as 1e-12 can't affect to the number (at least true in pytorch, verified in v1.5.0+).

* Revert "reduce 1e-12 to make difference between variables"

This reverts commit 33ec7215d22be59bf0012dea748f5cacc33a0fb8.

* reduce 1e-12 to make difference between variables

If 1e-12 is used to make a difference between t1 and t2, then later (t2-t1) might become 0 as 1e-12 can't affect to the number (at least true in pytorch, verified in v1.5.0+). Not sure whether 1e-7 or 1e-8 worked. But 1e-6 did.

mmdet3d/models/dense_heads/free_anchor3d_head.py

@@ -120,7 +120,7 @@ class FreeAnchor3DHead(Anchor3DHead):
                 # object_box_prob: P{a_{j} -> b_{i}}, shape: [i, j]
                 t1 = self.bbox_thr
                 t2 = object_box_iou.max(
-                    dim=1, keepdim=True).values.clamp(min=t1 + 1e-12)
+                    dim=1, keepdim=True).values.clamp(min=t1 + 1e-6)
                 object_box_prob = ((object_box_iou - t1) / (t2 - t1)).clamp(
                     min=0, max=1)