MelResNet spacing and typing adjustment (#725)

* spacing and typing adjustment

* remove import list
Co-authored-by: Ji Chen <jimchen90@devfair0160.h2.fair>

torchaudio/models/_wavernn.py

-from typing import Optional
-
 from torch import Tensor
 from torch import nn
 
@@ -99,9 +97,9 @@ class _MelResNet(nn.Module):
 
         Shape:
             - x: :math:`(N, S, T)`.
-            - output: :math:`(N, P, T-2*pad)`.
+            - output: :math:`(N, P, T - 2 * pad)`.
         where N is the batch size, S is the number of input sequence,
-        P is the number of ouput sequence, T is the length of input sequence.
+        P is the number of output sequence, T is the length of input sequence.
         """
 
         return self.melresnet_model(x)