[Fix] Support SiLU for Pytorch <= 1.5.0 (#158)

* support denoising unet for ddpm-based models

* revise known issues

* cvt mean/var_cfg to mean/var_mode

* add unit test

* support SiLU for pt<=1.5.0

* fix bug in pt version check

* fix another bug in pt version checking

mmgen/models/architectures/ddpm/modules.py

@@ -2,6 +2,7 @@
 from copy import deepcopy
 from functools import partial
 
+import mmcv
 import numpy as np
 import torch
 import torch.nn as nn
@@ -41,6 +42,9 @@ class SiLU(nn.Module):
 
     def __init__(self, inplace=False):
         super().__init__()
+        if torch.__version__ < '1.6.0' and inplace:
+            mmcv.print_log('Inplace version of \'SiLU\' is not supported for '
+                           f'torch < 1.6.0, found \'{torch.version}\'.')
         self.inplace = inplace
 
     def forward(self, x):
@@ -52,6 +56,9 @@ def forward(self, x):
             torch.Tensor: Tensor after activation.
         """
 
+        if torch.__version__ < '1.6.0':
+            return x * torch.sigmoid(x)
+
         return F.silu(x, inplace=self.inplace)