first commit; melgan support (perhaps bloated)

I copied this from my old and messy `tacotron-melgan` repo, where I had
previously experimented with running the two models in a pipeline.

I'll be moving over all the JIT pieces here and cleaning up when I can.

melgan-swpark/model_melgan/multiscale.py

@@ -0,0 +1,29 @@
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+
+from .discriminator import Discriminator
+from .identity import Identity
+
+
+class MultiScaleDiscriminator(nn.Module):
+    def __init__(self):
+        super(MultiScaleDiscriminator, self).__init__()
+
+        self.discriminators = nn.ModuleList(
+            [Discriminator() for _ in range(3)]
+        )
+        
+        self.pooling = nn.ModuleList(
+            [Identity()] +
+            [nn.AvgPool1d(kernel_size=4, stride=2, padding=1, count_include_pad=False) for _ in range(1, 3)]
+        )
+
+    def forward(self, x):
+        ret = list()
+
+        for pool, disc in zip(self.pooling, self.discriminators):
+            x = pool(x)
+            ret.append(disc(x))
+
+        return ret # [(feat, score), (feat, score), (feat, score)]