[Fix] update build loss api (open-mmlab#3587)

## Motivation

Use `MODELS.build` instead of `build_loss`

## Modification

Please briefly describe what modification is made in this PR.

Author: xiexinch

mmseg/models/decode_heads/decode_head.py

@@ -8,9 +8,9 @@
 from mmengine.model import BaseModule
 from torch import Tensor
 
+from mmseg.registry import MODELS
 from mmseg.structures import build_pixel_sampler
 from mmseg.utils import ConfigType, SampleList
-from ..builder import build_loss
 from ..losses import accuracy
 from ..utils import resize
 
@@ -140,11 +140,11 @@ def __init__(self,
         self.threshold = threshold
 
         if isinstance(loss_decode, dict):
-            self.loss_decode = build_loss(loss_decode)
+            self.loss_decode = MODELS.build(loss_decode)
         elif isinstance(loss_decode, (list, tuple)):
             self.loss_decode = nn.ModuleList()
             for loss in loss_decode:
-                self.loss_decode.append(build_loss(loss))
+                self.loss_decode.append(MODELS.build(loss))
         else:
             raise TypeError(f'loss_decode must be a dict or sequence of dict,\
                 but got {type(loss_decode)}')

mmseg/models/decode_heads/enc_head.py

@@ -9,7 +9,6 @@
 
 from mmseg.registry import MODELS
 from mmseg.utils import ConfigType, SampleList
-from ..builder import build_loss
 from ..utils import Encoding, resize
 from .decode_head import BaseDecodeHead
 
@@ -128,7 +127,7 @@ def __init__(self,
             norm_cfg=self.norm_cfg,
             act_cfg=self.act_cfg)
         if self.use_se_loss:
-            self.loss_se_decode = build_loss(loss_se_decode)
+            self.loss_se_decode = MODELS.build(loss_se_decode)
             self.se_layer = nn.Linear(self.channels, self.num_classes)
 
     def forward(self, inputs):

mmseg/models/decode_heads/vpd_depth_head.py

@@ -10,7 +10,6 @@
 
 from mmseg.registry import MODELS
 from mmseg.utils import SampleList
-from ..builder import build_loss
 from ..utils import resize
 from .decode_head import BaseDecodeHead
 
@@ -184,11 +183,11 @@ def __init__(
 
         # build loss
         if isinstance(loss_decode, dict):
-            self.loss_decode = build_loss(loss_decode)
+            self.loss_decode = MODELS.build(loss_decode)
         elif isinstance(loss_decode, (list, tuple)):
             self.loss_decode = nn.ModuleList()
             for loss in loss_decode:
-                self.loss_decode.append(build_loss(loss))
+                self.loss_decode.append(MODELS.build(loss))
         else:
             raise TypeError(f'loss_decode must be a dict or sequence of dict,\
                 but got {type(loss_decode)}')