Fix doc nightly doc CI (#3611)

Some changes at matplotlib 3.8.0 rejects torch.Tensor passed to `plot` function.

docs/requirements.txt

 Jinja2<3.1.0
-matplotlib
+matplotlib<=3.8
 pyparsing<3,>=2.0.2
 
 # C++ docs

examples/tutorials/additive_synthesis_tutorial.py

@@ -86,12 +86,12 @@ NUM_FRAMES = int(DURATION * SAMPLE_RATE)
 
 
 def plot(freq, amp, waveform, sample_rate, zoom=None, vol=0.1):
-    t = torch.arange(waveform.size(0)) / sample_rate
+    t = (torch.arange(waveform.size(0)) / sample_rate).numpy()
 
     fig, axes = plt.subplots(4, 1, sharex=True)
-    axes[0].plot(t, freq)
+    axes[0].plot(t, freq.numpy())
     axes[0].set(title=f"Oscillator bank (bank size: {amp.size(-1)})", ylabel="Frequency [Hz]", ylim=[-0.03, None])
-    axes[1].plot(t, amp)
+    axes[1].plot(t, amp.numpy())
     axes[1].set(ylabel="Amplitude", ylim=[-0.03 if torch.all(amp >= 0.0) else None, None])
     axes[2].plot(t, waveform)
     axes[2].set(ylabel="Waveform")

examples/tutorials/oscillator_tutorial.py

@@ -103,14 +103,14 @@ NUM_FRAMES = int(DURATION * SAMPLE_RATE)
 
 
 def show(freq, amp, waveform, sample_rate, zoom=None, vol=0.3):
-    t = torch.arange(waveform.size(0)) / sample_rate
+    t = (torch.arange(waveform.size(0)) / sample_rate).numpy()
 
     fig, axes = plt.subplots(4, 1, sharex=True)
-    axes[0].plot(t, freq)
+    axes[0].plot(t, freq.numpy())
     axes[0].set(title=f"Oscillator bank (bank size: {amp.size(-1)})", ylabel="Frequency [Hz]", ylim=[-0.03, None])
-    axes[1].plot(t, amp)
+    axes[1].plot(t, amp.numpy())
     axes[1].set(ylabel="Amplitude", ylim=[-0.03 if torch.all(amp >= 0.0) else None, None])
-    axes[2].plot(t, waveform)
+    axes[2].plot(t, waveform.numpy())
     axes[2].set(ylabel="Waveform")
     axes[3].specgram(waveform, Fs=sample_rate)
     axes[3].set(ylabel="Spectrogram", xlabel="Time [s]", xlim=[-0.01, t[-1] + 0.01])

examples/tutorials/subtractive_synthesis_tutorial.py

@@ -227,7 +227,7 @@ def plot_waveform(magnitudes, filtered, sample_rate):
 
     _, ax = plt.subplots(1, 1, sharex=True)
     ax.vlines(offsets, 0, nyquist, color="gray", linestyle="--", linewidth=0.8)
-    ax.plot(mag_x.T, mag_y.T, color="gray", linewidth=0.8)
+    ax.plot(mag_x.T.numpy(), mag_y.T.numpy(), color="gray", linewidth=0.8)
     ax.specgram(filtered, Fs=sample_rate)
     return Audio(filtered, rate=sample_rate)