FFmpeg 7.1 deprecated AVCodec::ch_layouts

[scotts]

When compiling against FFmpeg 7.1, we get deprecation warnings because AVCodec has deprecated the field ch_layouts. We'll need to deal with it here: https://github.com/pytorch/torchcodec/blob/da3eddaa6746d093b09ad1413a42d7244686c58e/src/torchcodec/_core/FFMPEGCommon.cpp#L111-L133 One wrinkle is that this code is already dealing with FFmpeg > 4 versions switching from channel_layout to ch_layouts, so this will likely need to become a three-armed #if.