1. Add image_ops.is_jpeg Op to decide if a input string is a jpeg string or not.

2. Change tfexample_decoder in slim/objection_detection to accept different JPEG decompression method.
Defaults to ""/None which maps to a system-specific default. Currently valid values are ["INTEGER_FAST", "INTEGER_ACCURATE"]. The hint may be ignored (e.g., the internal jpeg library changes to a version that does not have that specific option.)

PiperOrigin-RevId: 185486653