[Feature] Add MonoFlex Coder (#1115)

* add monoflex coder

* fix coder

* fix comments

* change variable name

* fix typos

* put encode alpha in coder

* change variable name

* change alpha to local yaw

* fix roty to yaw

* fix annotations

* fix comments

* fix return elements

* fix comments

* fix comments

mmdet3d/core/bbox/coders/__init__.py

@@ -5,6 +5,7 @@ from .centerpoint_bbox_coders import CenterPointBBoxCoder
 from .delta_xyzwhlr_bbox_coder import DeltaXYZWLHRBBoxCoder
 from .fcos3d_bbox_coder import FCOS3DBBoxCoder
 from .groupfree3d_bbox_coder import GroupFree3DBBoxCoder
+from .monoflex_bbox_coder import MonoFlexCoder
 from .partial_bin_based_bbox_coder import PartialBinBasedBBoxCoder
 from .pgd_bbox_coder import PGDBBoxCoder
 from .point_xyzwhlr_bbox_coder import PointXYZWHLRBBoxCoder
@@ -13,5 +14,6 @@ from .smoke_bbox_coder import SMOKECoder
 __all__ = [
     'build_bbox_coder', 'DeltaXYZWLHRBBoxCoder', 'PartialBinBasedBBoxCoder',
     'CenterPointBBoxCoder', 'AnchorFreeBBoxCoder', 'GroupFree3DBBoxCoder',
-    'PointXYZWHLRBBoxCoder', 'FCOS3DBBoxCoder', 'PGDBBoxCoder', 'SMOKECoder'
+    'PointXYZWHLRBBoxCoder', 'FCOS3DBBoxCoder', 'PGDBBoxCoder', 'SMOKECoder',
+    'MonoFlexCoder'
 ]