refactor(wavernn): remove duplicate Stretch2d

I checked that the implementations are the same

TTS/vocoder/models/wavernn.py

@@ -17,6 +17,7 @@
 from TTS.utils.audio.numpy_transforms import mulaw_decode
 from TTS.vocoder.datasets.wavernn_dataset import WaveRNNDataset
 from TTS.vocoder.layers.losses import WaveRNNLoss
+from TTS.vocoder.layers.upsample import Stretch2d
 from TTS.vocoder.models.base_vocoder import BaseVocoder
 from TTS.vocoder.utils.distribution import sample_from_discretized_mix_logistic, sample_from_gaussian
 
@@ -66,19 +67,6 @@ def forward(self, x):
         return x
 
 
-class Stretch2d(nn.Module):
-    def __init__(self, x_scale, y_scale):
-        super().__init__()
-        self.x_scale = x_scale
-        self.y_scale = y_scale
-
-    def forward(self, x):
-        b, c, h, w = x.size()
-        x = x.unsqueeze(-1).unsqueeze(3)
-        x = x.repeat(1, 1, 1, self.y_scale, 1, self.x_scale)
-        return x.view(b, c, h * self.y_scale, w * self.x_scale)
-
-
 class UpsampleNetwork(nn.Module):
     def __init__(
         self,