Use `at::parallel_for` in lfilter core loop (#1557)

torchaudio/csrc/lfilter.cpp

@@ -16,13 +16,13 @@ void host_lfilter_core_loop(
   scalar_t* output_data = padded_output_waveform.data_ptr<scalar_t>();
   const scalar_t* input_data = input_signal_windows.data_ptr<scalar_t>();
   const scalar_t* a_coeff_flipped_data = a_coeff_flipped.data_ptr<scalar_t>();
-  for (int64_t i_batch = 0; i_batch < n_batch; i_batch++) {
-    for (int64_t i_channel = 0; i_channel < n_channel; i_channel++) {
+
+  at::parallel_for(0, n_channel * n_batch, 1, [&](int64_t begin, int64_t end) {
+    for (auto i = begin; i < end; i++) {
+      int64_t offset_input = i * n_samples_input;
+      int64_t offset_output = i * n_samples_output;
+      int64_t i_channel = i % n_channel;
       for (int64_t i_sample = 0; i_sample < n_samples_input; i_sample++) {
-        int64_t offset_input =
-            ((i_batch * n_channel) + i_channel) * n_samples_input;
-        int64_t offset_output =
-            ((i_batch * n_channel) + i_channel) * n_samples_output;
         scalar_t a0 = input_data[offset_input + i_sample];
         for (int64_t i_coeff = 0; i_coeff < n_order; i_coeff++) {
           a0 -= output_data[offset_output + i_sample + i_coeff] *
@@ -31,7 +31,7 @@ void host_lfilter_core_loop(
         output_data[offset_output + i_sample + n_order - 1] = a0;
       }
     }
-  }
+  });
 }
 
 void cpu_lfilter_core_loop(