Fix compilation error when there is no libjpeg

[cdzhan]

Fix #8339. The png_structp and j_decompress_ptr are defined in png.h and jpeglib.h and there seems no need to define fetch_jpeg_exif_orientation when libraries are not found?

[pytorch-bot]

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[cdzhan]

cc @NicolasHug

[vfdev-5]

Thanks for the fix @cdzhan, lgtm

The png_structp and j_decompress_ptr are defined in png.h and jpeglib.h and there seems no need to define fetch_jpeg_exif_orientation when libraries are not found?

Yes, we can allow not to define these functions when jpeg/png are not found such that for example decode_jpeg
function will be defined by

vision/torchvision/csrc/io/image/cpu/decode_jpeg.cpp

Lines 8 to 16 in ef2920c

	#if !JPEG_FOUND
	torch::Tensor decode_jpeg(
	const torch::Tensor& data,
	ImageReadMode mode,
	bool apply_exif_orientation) {
	TORCH_CHECK(
	false, "decode_jpeg: torchvision not compiled with libjpeg support");
	}
	#else

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