Migrate TimeStretch and AmplitudeToDB to torch.nn.Module (#456)

* AmplitudeToDB to torch.nn.Module

* TimeStretch use torch.nn.Module

torchaudio/transforms.py

@@ -137,7 +137,7 @@ class GriffinLim(torch.nn.Module):
                             self.normalized, self.n_iter, self.momentum, self.length, self.rand_init)
 
 
-class AmplitudeToDB(torch.jit.ScriptModule):
+class AmplitudeToDB(torch.nn.Module):
     r"""Turn a tensor from the power/amplitude scale to the decibel scale.
 
     This output depends on the maximum value in the input tensor, and so
@@ -157,7 +157,7 @@ class AmplitudeToDB(torch.jit.ScriptModule):
         self.stype = stype
         if top_db is not None and top_db < 0:
             raise ValueError('top_db must be positive value')
-        self.top_db = torch.jit.Attribute(top_db, Optional[float])
+        self.top_db = top_db
         self.multiplier = 10.0 if stype == 'power' else 20.0
         self.amin = 1e-10
         self.ref_value = 1.0
@@ -592,7 +592,7 @@ class ComputeDeltas(torch.nn.Module):
         return F.compute_deltas(specgram, win_length=self.win_length, mode=self.mode)
 
 
-class TimeStretch(torch.jit.ScriptModule):
+class TimeStretch(torch.nn.Module):
     r"""Stretch stft in time without modifying pitch for a given rate.
 
     Args:
@@ -610,8 +610,7 @@ class TimeStretch(torch.jit.ScriptModule):
 
         n_fft = (n_freq - 1) * 2
         hop_length = hop_length if hop_length is not None else n_fft // 2
-        phase_advance = torch.linspace(0, math.pi * hop_length, n_freq)[..., None]
-        self.phase_advance = torch.jit.Attribute(phase_advance, torch.Tensor)
+        self.phase_advance = torch.linspace(0, math.pi * hop_length, n_freq)[..., None]
 
     def forward(self, complex_specgrams, overriding_rate=None):
         # type: (Tensor, Optional[float]) -> Tensor