test infrastructure: fft: Align claimed data type with CUDA in backward test

[fengyuan14]

🚀 The feature, motivation and pitch

XPU data types supported at native are more than CUDA. Like we always support BF16, but CUDA in some operators doesn't support BF16. So we have got a handle in test infrastructure to add BF16 to our claimed data types,
backward_dtypesIfXPU = backward_dtypesIfCUDA + bfloat16.

But in fft operators, we should claim not supporting BF16. The existing assumption in test infrastructure leads to some fft unit test failures,

    "test_dtypes_fft_fft2_xpu",
    "test_dtypes_fft_fft_xpu",
    "test_dtypes_fft_fftn_xpu",
    "test_dtypes_fft_hfft2_xpu",
    "test_dtypes_fft_hfft_xpu",
    "test_dtypes_fft_hfftn_xpu",
    "test_dtypes_fft_ifft2_xpu",
    "test_dtypes_fft_ifft_xpu",
    "test_dtypes_fft_ifftn_xpu",
    "test_dtypes_fft_ihfft2_xpu",
    "test_dtypes_fft_ihfft_xpu",
    "test_dtypes_fft_ihfftn_xpu",
    "test_dtypes_fft_irfft2_xpu",
    "test_dtypes_fft_irfft_xpu",
    "test_dtypes_fft_irfftn_xpu",
    "test_dtypes_fft_rfft2_xpu",
    "test_dtypes_fft_rfft_xpu",
    "test_dtypes_fft_rfftn_xpu",

Alternatives

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Additional context

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