remove duplicate codes in Vocoder MelGAN (#375)

Author: Yuan-ManX

models/vocoders/gan/generator/melgan.py

@@ -3,9 +3,6 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-import torch
-import torch.nn as nn
-import torch.nn.functional as F
 import torch
 import torch.nn as nn
 import torch.nn.functional as F