Jpeg remove source ctx

[NicolasHug]

The source_ctx_t is not needed, our source is always a tensor of uint8 so it fits with the builtin pointer-based libjpeg mechanism by just setting

  jpeg_ctx.src->bytes_in_buffer = len;
  jpeg_ctx.src->next_input_byte = data;

We were in fact never relying on the extra data and len fields of the source_ctx_t struct.

[pytorch-bot]

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