Exporting the operator stft to ONNX opset version 9 is not supported.

Hi, I try exporting the process of feature extraction to onnx:

import torch
import torchaudio

model = torchaudio.transforms.MelSpectrogram()
x = torch.randn(1, 16000)
torch.onnx.export(model, x, 'tmp.onnx', input_names=['input'], output_names=['output'])

and get:

RuntimeError: Exporting the operator stft to ONNX opset version 9 is not supported. Please open a bug to request ONNX export support for the missing operator.

So will torchaudio add supports operators used in torchaudio.transforms module in the future? You see that exporting the process of feature extraction and the neural network together will be very convenient.

Thanks!

Start with torchaudio.transforms.MelSpectrogram and the torch.onnx.export call shown in the issue. Check the ONNX opset 9 handling for the stft operator and related torchaudio.transforms operators; done means the feature-extraction process exports successfully without the reported unsupported-operator error.