Fix view size error when backpropagating through lfilter

[yoyolicoris]

This commit solves the view size error when the gradient tensor propagates to the filter is not in a contiguous format.

The bug can be reproduced by the following script (provided by @forgi86) using the latest version of torchaudio.

import torch
import torchaudio.functional

order = 5
in_channels = 10
seq_len = 1000
batch_size = 32

b_coeff = torch.randn((in_channels, order)) * 0.05
b_coeff = b_coeff.requires_grad_(True)
a_coeff = torch.randn((in_channels, order - 1)) * 0.05
a_coeff = a_coeff.requires_grad_(True)

ones = torch.ones_like(a_coeff[..., :1])
a_coeffs_with_ones = torch.cat((ones, a_coeff), dim=-1)

u = torch.randn((batch_size, in_channels, seq_len))

y = torchaudio.functional.lfilter(u, a_coeffs_with_ones, b_coeff, clamp=False, batching=True)
y = y.transpose(-2, -1).reshape(batch_size, in_channels // 2, 2, seq_len)

loss = torch.sum(y**2)
loss.backward()

RuntimeError: view size is not compatible with input tensor's size and stride (at least one dimension spans across two contiguous subspaces). Use .reshape(...) instead.

[pytorch-bot]

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❌ 4 New Failures

As of commit 8745e8c with merge base b829e93 ():

NEW FAILURES - The following jobs have failed:

• Unit-tests on Linux GPU / tests (3.8, 11.8) / linux-job (gh)
  ##[error]The operation was canceled.
• Unit-tests on Linux GPU / tests (3.9, 11.8) / linux-job (gh)
  ##[error]The operation was canceled.
• Unittests on Windows CPU / unittests-windows-cpu / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Unittests on Windows GPU / unittests-windows-gpu / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

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