fix autograd maybe has problem when using inplace operation

deepspeech/modules/conformer_convolution.py

@@ -122,7 +122,7 @@ class ConvolutionModule(nn.Layer):
 
         # mask batch padding
         if mask_pad is not None:
-            x.masked_fill_(mask_pad, 0.0)
+            x = x.masked_fill(mask_pad, 0.0)
 
         if self.lorder > 0:
             if cache is None:
@@ -156,7 +156,7 @@ class ConvolutionModule(nn.Layer):
 
         # mask batch padding
         if mask_pad is not None:
-            x.masked_fill_(mask_pad, 0.0)
+            x = x.masked_fill(mask_pad, 0.0)
 
         x = x.transpose([0, 2, 1])  # [B, T, C]
         return x, new_cache