Add fallback for USE_ONEMKL=OFF

Author: CuiYifeng

src/ATen/native/xpu/SpectralOps.cpp

@@ -3,6 +3,7 @@
 #else
 #include <ATen/native/Resize.h>
 #include <ATen/ops/_fft_c2c_native.h>
+#include <ATen/ops/_fft_c2r_native.h>
 #endif // USE_ONEMKL
 
 namespace at::native {
@@ -49,18 +50,33 @@ Tensor _fft_c2r_xpu(
     int64_t last_dim_size) {
   TORCH_CHECK(self.is_complex());
 
+#if defined(USE_ONEMKL)
   return native::xpu::_fft_c2r_mkl(self, dim, normalization, last_dim_size);
+#else
+  Tensor out_cpu = native::_fft_c2r_mkl(
+      self.to(Device(at::kCPU)), dim, normalization, last_dim_size);
+  return out_cpu.to(Device(at::kXPU));
+#endif // USE_ONEMKL
 }
 
-Tensor _fft_c2r_xpu_out(
+Tensor& _fft_c2r_xpu_out(
     const Tensor& self,
     IntArrayRef dim,
     int64_t normalization,
     int64_t last_dim_size,
     Tensor& out) {
   TORCH_CHECK(self.is_complex());
 
-  return native::xpu::_fft_c2r_mkl_out(self, dim, normalization, last_dim_size, out);
+#if defined(USE_ONEMKL)
+  return native::xpu::_fft_c2r_mkl_out(
+      self, dim, normalization, last_dim_size, out);
+#else
+  Tensor out_cpu = native::_fft_c2r_mkl(
+      self.to(Device(at::kCPU)), dim, normalization, last_dim_size);
+  at::native::resize_output(out, out_cpu.sizes());
+  out.copy_(out_cpu);
+  return out;
+#endif // USE_ONEMKL
 }
 
 } // namespace at::native